Trying to keep a Fiero on the road, it's ODTAA (one d***n thing after another). I now have another intermittent problem: is it fuel supply or exhaust blockage or something else?

Quite simply when driving my 88 Formula (2.8 auto) at around 40-45 or above (ie in 'top' gear), when I push fairly firmly on the gas to accelerate, I sometimes get a serious hesitation, almost as if the engine is refusing to rev (the tach actually drops, or rises and then drops), and the car barely accelerates at all. Feels as if it's being strangled. If I really floor the pedal, the transmission kicks down, of course, and away I go.

In Park or Neutral, the engine idles perfectly, and revs up absolutely smoothly with no hesitation, stutter or pop. The problem seems to arise only under load. As well as the acceleration problem, I've noticed that when stationary but in Drive (eg at traffic lights) the idle is occasionally a bit rough/erratic: it idles at around 1000, but the revs fluctuate downwards to about 700 (hard to describe, but it feels as if the engine is fighting the transmission!). First few times I noticed, I thought the engine was going to stall, but it hasn't (yet). Don't know if there's any link between these two symptoms...

I'm not sure what to check first, or in what order, to pin down the problem. I'm getting no codes. I wonder if it's fuel-supply problems, some electrical or sensor gremlin, or exhaust obstruction? By way of background, I've recently put some injector cleaner through, as I did have a one-off cranking/starting problem (flooding?) which however mysteriously cured itself . The electrics should be OK: I have a new alternator (old one died), a good battery, and an extra earth strap as recommended on the Forum. In the auto trans, the TCC solenoid is currently disconnected as it wasn't unlocking (I am about to fit a new one). And in terms of the exhaust, have NO catalytic converter - when the internal stuff disintegrated years ago, it was replaced with a 'cherry bomb' ( but I do sometimes wonder if there's any debris still sitting in the rear muffler...).

Anyone got any ideas?

br1anstorm

Well, the first thing to try, is disconnecting the tach filter. Apologies if you've already been down this road, and it may not solve any of your problems - but it 's probably the first thing I'd try (start with cheap & reversible).

Second thing to check is the timing - although you've possibly already checked it.

Third thing I'd probably look at is to see if the idle air passages are dirty.

[This message has been edited by StuGood (edited 12-02-2008).]

I'm gonna throw a guess towards a sluggish EGR. I expect the EGR is wide open as you lightly cruise along, and when you hit the gas - the EGR should quickly close.

Thanks - I haven't had a chance yet to start messing around under the hood or testing anything (the weather has been too foul for me to work on the car which is parked outside). But I still use the car daily, and I've noticed something else which may narrow down the diagnosis. The problem only arises when the engine is hot.

When the engine is cold and for the first few miles of a trip, it idles well, revs up smoothly and the acceleration is prompt and good (ie normal!).

But when the engine is warmed up, that's when the problem appears. It still revs smoothly when out of gear. When under load, steady and gentle pressure on the gas gets the revs and the speed up gradually. But then heavier pressure on the gas pedal when going at say around 40mph causes the revs to drop quite noticeably, and the car simply won't accelerate (it really does feel as if it's being held back by an invisible hand!). Is this confirmation of an EGR problem? (If so that would surely occur irrespective of temperature?). Or is it some sort of fuel/mixture problem (flooding???). Once when I almost floored it, I think I heard it backfire - just before it kicked down a gear, the revs shot up and away the car went....

And - I still don't know if this is related - when idling cold, it is steady at about 1100 in P or N, and rock steady at 950/1000 in D or R. But when idling warm/hot and in D (eg at traffic lights), then the revs fluctuate according to a distinct pattern: it idles at around 1000 but every 10/15 seconds, the revs drop for 5 secs, then up again.

Seems very weird to me....

Ordinarily, I would never speak for another man. But I've seen Friz post the same thing so many times it ain't funny, hehe. Figured I'd save him a few steps.

quote Originally posted by Frizlefrak: They need 3 things to run correctly; 1. Suitable compression 2. The correct mixture of fuel and air for a given engine speed and load. 3. A suitably strong spark delivered at the correct time. If those 3 conditions are met, they run. This applies to any internal combustion engine built in the last 120 years. I'm assuming compression is good on the engine, but maybe I shouldn't. How many pounds of compression on each cylinder? Let's look at pressure and variance. That said, what is your fuel pressure under each of these operating conditions? I don't want to know if it's getting fuel....it will start, so that much is obvious....I want to know EXACTLY how many pounds under each circumstance. The TBI system needs between 9-13 psi to run correctly, but really, any less than 10 psi and performance will fall off dramatically. Low fuel pressure may be sufficient to rev with no load on the engine...but once it's under strain, it falls on it's face. There is no schrader valve on a TBI system, which means a low pressure gauge, T fittings, hose, and clamps. No fun to check, but essential. If you can route the gauge over the roof and tape it to the windshield, you should be able to read it as you drive. I also want to know if the injector is spraying a nice, conical mist under all conditions....or if it may need to be serviced. This can be tough to verify moving without a chassis dyno....any access to one? If not, a buddy who's willing to ride in the trunk? (just kidding )While were at it, how many inches of vacuum at idle? Does the needle hold steady or waver? These tests will either eliminate the mechanical health and fuel systems as suspects, or make us dig deeper. Either situation is OK, it means we're one step closer. And remember to troubleshoot, don't guess. Throwing parts at a problem to fix it can get very expensive in a hurry. Happy wrenching, and post your results.

I was having a problem exactly like the one you describe here. It almost seemed as though the pedal had a "sweet spot" you had to dial into in order to get the car to rev. I could get mine to 70 mph but it took 3 x longer than normal and I had to keep fondling the pedal to do it. It idled great, revved great at a standstill, but as soon as I put a load on it, BLEHHHHHhhhhhhhhhh-cough-cough-sputter-gag-ka-put. No power.

It ended up being my fuel pump. Friz told me to verify fuel pressure and that anything between 8-13 psi was good for a 2.5L. (45 psi for the 2.8L.) I installed a fuel pressure gauge and came up with 7 psi. I wanted to believe 7 psi was just as good as 8 psi but I changed the fuel pump anyway and the car now runs like a scalded dog.

[This message has been edited by Brocephus (edited 12-08-2008).]

All advice from wherever it comes is appreciated! That Friz post (which I hadn't seen elsewhere or when I searched) is good sense and practical logic. I like that. Only problem is, it assumes a workshop-full of testing tools...

Compression? I have no gauges, and let's face it, trying to get access to the three forward spark plugs (under the window) is a "do I really have to do this" kind of task. But I have no reason to suspect my low-mileage engine has a compression problem.

Vacuum - ditto on gauges...and there seem to be a lot of places where there could be leaks/problems. I can't see any obviously perished, cracked or missing vacuum hoses - but that doesn't mean a great deal.

Injector health and chassis dyno - can't even begin to contemplate such things. But I did put a dose of injector-cleaner into the tank recently...

Fuel pressure - there is a schrader valve on the V6 and I'm hoping soon to lay hands on a gauge for this (they are hard to find over here and very expensive, but thanks to a Fiero-Forum friend, one is en route to me!). Meanwhile the simple test (open cap and press on little pin in valve) suggests that the fuel is coming through OK - but of course I can't monitor pressure under different speed/load conditions.

I guess what I'm hoping is that with a clear enough pattern of symptoms, someone might be able to suggest the most likely cause.

In this respect, I am lucky I have a 2.5L, hehe. My spark plugs are on top of the motor and readily accessible. I only have 6 total vacuum lines, so when there's suspicion of a vaccum leak, I just change them all.

I got lucky in the way of guages as my dad gave up working on cars and handed his tools to me. I have several vacuum and fuel pressure gauges as well as a couple of compression gauges. I even have a Starrett Micrometer, in its original wooden box, from 1948!

Best of luck with the dignosis and post up your results. :-)

I'm guessing fuel pressure or plugged cat.
Have you changed the fuel filter?
Get a gauge on it and make sure it's good.
Might also read this post https://www.fiero.nl/forum/Forum2/HTML/095961.html
or any others regarding plugged cats.

Or as a cat test if your down pipe coupler after the Y is not to rusty you could open the exhaust at that point and see how it runs.
If you soak the bolts, clean the threads with a wire brush and then heat the threads and nut they often unscrew fairly well.

[This message has been edited by Dodgerunner (edited 12-09-2008).]

Dodgerunner, maybe you are getting closer to nailing this mystery. At least your comments have prompted me into some further thoughts.

I need to test my fuel pressure properly anyway, as you suggested when I had a one-off starting problem a month or so ago (see the thread here https://www.fiero.nl/forum/Forum2/HTML/096125.html ). Soon I will be able to do so, thanks to another PFF member, montegut. He kindly brought a gauge over from the US and passed it on to a friend in London. Now I have to get it sent on up here to Scotland! Also I haven't changed the fuel filter - so I'll start the search for a replacement one.

Meanwhile I've been pondering on the cat/muffler angle. I don't have a cat, BUT...there's some history which may be relevant. Some 8-9 years ago, the original cat disintegrated and the debris did block things up. A local workshop found an aftermarket performance muffler (we call it a cherry-bomb) which would fit into the space, so they threw out the cat and fitted that as a replacement.

Thanks to storage and lack of use, that replacement cherry-bomb muffler rusted over the past 3 years. The rest of the exhaust system from there back to the outlets however still looks OK, with relatively little rust. So when we put the car back on the road, we fitted another cherry-bomb. Engine seemed to run all right, and passed its emission tests (the limits are not too strict here).

Now here's the thing I've been wondering about. When the original cat disintegrated, most of the broken bits were obviously inside it - and that's essentially where the obstruction was. But now I'm wondering: might any of the debris have gone on down the exhaust and lodged in the rear muffler? It may have been sitting there for years, but could it now for some reason (because I'm exercising the car after a long lay-up) be blocking the exhaust flow and causing this performance/acceleration problem. Just a theory...

Is there any way of checking/testing whether there's any stuff obstructing that rear muffler, and "clearing it out" (without, of course, chopping it open, which would then mean having to replace it anyway)? I presume there are baffles inside that rear muffler, so I can't just poke a stick or piece of wire up the exhaust pipe!

I'm reluctant to destroy the rear muffler just to see if it is/was OK, because there's no readily available OEM replacement. Shipping an exhaust system including rear muffler from the US would cost a fortune. The alternative would be to get a new system specially fabricated locally. I don't want to do that unless or until I'm quite sure the existing system is indeed blocked and causing the problem....

Is there any quick and dirty way of checking that the rear muffler isn't clogged? Seems it's not advice, but inspiration, I need now!

A quick follow-up. If these problems only appear when the engine is warmed up, might they be related to the electronic control circuits which change from open-loop to closed-loop (or vice versa...) once operating temp is reached? I don't really understand the electronics and/emissions systems - it's all black magic to me.

But it seems more than coincidence that my car's problems appear only when it's warmed up. Most fuel/starting/idling issues arise when the engine is cold, and normally things work better once everything is up to operating temperature. With mine, it's the opposite - easy starting, and runs smoothly when cold, but then misbehaves once operating temp is reached.

Of course this takes us away from the theory of a blocked exhaust/muffler. But is it another clue to the cause of the problem? I'm just trying to do some logical thinking in order to figure out where to look and what to test...

I still haven't solved this problem, and it seems to be getting worse. No diagnostic codes. And fuel consumption seems to be horrible (haven't worked out mpg, and I know the fuel gauge isn't very accurate, but 60 miles has used half a tank of gas).

I have visually checked all the vacuum hoses and sprayed WD40 around them. All seem OK. I have tested the TPS with a meter - the figures seem to be smooth all the way from closed throttle to wide-open. I've stuck my thumb over the hole in the air-intake to check the IAC, and the engine stalled. When I push the EGR valve diaphragm up with my fingers while the car idles, the revs drop - which I think is what's supposed to happen.

The problem seems to be fuel/air related (or could it be ignition timing?). Haven't yet got hold of a fuel pressure gauge. But the really distinctive symptom is that - once the engine is warm (and transmission in Park) if I ease open the throttle gradually and steadily, the revs rise smoothly all the way to the red line. But if I 'blip' the throttle hard, or whack it open wide very quickly (I do this by hand using the throttle lever/pivot on the throttle body), the revs drop (or don't rise), the engine runs horribly rough, and I can provoke it to backfire. But there's no smoke of any colour out of the exhausts.

Reminds me a bit of what happens in older, simpler cars if you run a warm engine with the manual choke out (ie very rich) - when acceleration effectively floods the cylinders.

But without vacuum gauge, fuel pressure gauge or other similar kit, I really don't know what to check or test next. Any more ideas, anyone?

Take the O2 sensor out of the manifold and try to start it. If the rear exhaust is restricted it should now run because the exhaust now has a place to exit. Don't leave it out. Just check for restriction this way.

Thanks rpoZ25... I did wonder if anything might be restricting the exhaust, but I'm pretty sure it's clear. The car starts, runs and accelerates fine when cold. The problem only starts to show once the engine is warm. The cat was removed many years ago, and it just seems unlikely that any bits of debris from it would only now be causing a blockage further back. There's certainly lots of exhaust gas escaping through both rear outlets - the usual bit of pale smoke initially on startup on cold days, but then clear gases (ie no dark oily smoke, nor clouds of unburnt fuel or moisture). I have no idea what the back-pressure is, but I reckon the exhaust gases can escape easily enough without having to find an exit though the (rather small) O2 sensor hole!

I did wonder if the inside of the tailpipes was a bit more sooty than usual (rich mixture?) - but to be honest I don't know what's normal as I don't peer inside the tailpipes all that often...

If the basic tests checks out, such as good fuel pressure, strong spark, proper timing, then would a bad / failing O2 sensor be the cause of the problem ?

The problems only appear when the engine is warmed up, quite possible once the engine enters closed-loop mode.
The ECM computer does not start using the O2 sensor until the engine is in closed-loop mode.
Rich mixture and excessive fuel consumption can be caused by a bad O2 sensor.

The ECM computer reads two temp. sensors on the V6, which can also lead to a rich mixture.
Connecting the ECM to a scanner could help you locate a failing sensor.
Just my thoughts after reading prior posts.

Interesting thought, StockGT... but wouldn't a bad O2 sensor, or for that matter a bad temp sensor, throw a code? Needless to say, I haven't got a scanner - and as far as I know there's no-one with a scanner within a few thousand miles of here! Is there anything I can do to test the 02 sensor?

br1anstorm

Hi Br1anstorm...

I'm the infamous "Friz" that my friend Brocephus alluded to earlier. Sorry I'm so late to the party, but we've had a family emergency the last week, which is still ongoing, and I'm not getting on here as much as before, and probably won't for a while. I'm leaving a lot of my former posting in Brocephus' very capable hands for the time being. The dude is a seriously good troubleshooter / wrencher.

I will try to keep in mind that tools and facilities are at a premium where you're at....and adjust my diagnosing accordingly.....but some tools are essential. I'm glad a forum member has a fuel pressure gauge on the way.

I troubleshoot always in the same order, as Brocephus enumerated above, for several reasons. 1st, I feel like the order is logical, in that the mechanical health of the engine in question is determined immediately. No point in further diagnosing an engine that has 35 psi of compression on a cylinder. I don't really think that's your case, but you get the picture. Second, I'm just anal about doing things the same way every time....my wife says I was toilet trained at gunpoint. Not sure if that is true...will ask my mom and get back to you. Incidentally, a plugged exhaust will affect your compression readings....they will be fine in a static test (all plugs removed), but deficient across the board in a running test, as the engine can't exhale. Yes, I understand that accessing the rear 3 plugs is about as much fun as a prostate exam....but a man's gotta do what a man's gotta do.

Next, I like to move onto the fuel system. The fuel system, even in an EFI car, is remarkably simple and easy to diagnose. Once we know that the engine is mechanically healthy, we need to know if it is being fed correctly or if it's malnourished. A fuel pressure test is the first bullet in our arsenal here. Your V6 should have around 45 psi of pressure....and it should remain constant across the operational spectrum of the engine. I know tools are a little harder to come by in the UK, and I will stay tuned until your gauge arrives from England and you can properly test the pressure. As for the runs good cold but not hot issue....the pump could be responsible.....heat causes electrical resistance, and could cause a marginal pump to work fine cold, but start to give out after a few minutes. I don't really think this is your issue, but I'd like to rule it out. Here's what you should see...



The pressure should hold for several minutes after you remove power from the fuel pump. If it doesn't, please post....and we can go throught the sequence of determining why not (leaking injectors, pressure regulator, pulsator, check valve, etc.)

Finally, if the engine is breathing good and well fed, that leaves us with ignition / emissions. Since you're not getting any DTC's stored in the computer, the sensors (emissions) and associated wiring aren't giving the ECM any information that it finds distasteful. So, once your compression and fuel pressure are given a clean bill of health, I want to start looking at your ignition system....particularly the ignition module and associated wiring. Remember,.....heat causes electrical resistance....and like the fuel pump, a marginal module may perform fine when cold....and get flaky when it gets hot. Also, check under the module and ensure that it has a light coating of module grease under it....this is essential. Here in the colonies, we can take our modules out and AutoZone will test them for free....do you have any place in Scotland where you can have this done? I also would like you to test the EGR valve for proper operation.

Sooo....in short....I want to know.

1. Your compression readings for each cylinder, static and running.
2. Your fuel pressure readings at key on, idle, and full throttle (take a passenger to look over their shoulder )
3. If your ignition module tests out correctly and has the requisite coating of grease.

I know it's a lot to ask, and I apologize for being so long winded, but I like to be thorough. Approach it methodically, and you will track down the gremlin causing this and eradicate it.

Happy Wrenching...

Dan

[This message has been edited by Frizlefrak (edited 12-23-2008).]

Hello Dan/Friz

I think I should be calling you Father Christmas! You have just delivered a whole sackful of helpful advice....

Thanks for taking the time and trouble to post when you have other concerns on your mind. Hope the domestic/family issues aren't too serious.

I've lined up to use a friend's workshop after the Christmas/New Year break, and he has some of the necessary toys like a compression tester, so with your advice to refer to we'll see what we can find. Your systematic troubleshooting guide fits with my own "simple-and-ignorant-but-logical" approach. As my friend says, "All you have to know about engines is that they Suck, Squeeze, Bang and Blow".

Oh and by the way, there's no copyright on ODTAA - feel free to use it whenever appropriate!

Thanks again - stand by for a further sitrep in a week or two...

br1anstorm

quote Originally posted by br1anstorm: As my friend says, "All you have to know about engines is that they Suck, Squeeze, Bang and Blow". Oh and by the way, there's no copyright on ODTAA - feel free to use it whenever appropriate!

I haven't heard the S,S,B,&B in a while But yes, that's essentially all they do. Just a matter of figuring out which one they are having trouble doing & why.

And thanks for the well wishes....we have a family member in the hospital with end stage emphysema. She's 89 years old so it's not unexpected, but we're still going through a rough patch. I appreciate the kind words.

Anyhow, I do miss coming on here and troubleshooting. I take great pleasure in helping some get or keep their Fiero on the road. Whenever I read how someone is having trouble I always wish I could be there to help them in person. And even when I don't hit the correct diagnosis right off, this board is full of knowledgeable folks who do. It's very gratifying to see someone hit the key and have it run like it should. I will live vicariously through Brocephus and the others on here for the meantime.

Glad to hear you've got a shop and some tools to help you out. Good luck troubleshooting, and let us know your progress.

Memo to self, number #1 - CHECK THE BASICS!

Memo to self, number #2 - IF YOU START GUESSING, CHANCES ARE YOU'LL GUESS WRONG

This is the point where at great embarrassment to myself, but out of courtesy and appreciation to those good people who offered advice and expertise on this thread (thanks again guys!), I have to reveal that we've discovered what was actually wrong with my Fiero....

I took the car out on a run last weekend in order to warm it up with the intention of (a) testing the temp sensor and (b) trying to remove and test the O2 sensor. Started OK, but the poor acceleration and rough idle when warm was noticeably worse, and five miles down the road after a give-way junction, the car died and wouldn't restart. My workshop-owning friend trailered it back, and we set about checking it out.

What did we discover? Terminals inside distributor cap virtually disintegrated. Centre pin and spring crumbled to pieces. Rotor arm looking the worse for wear (and the plastic cracked when we forced it off: it was stuck to the - rusty - spindle). Evidently damp had got inside the distributor when the car was sitting un-used for a couple of years in the garage, and corroded the exposed metals.

Oh the shame! I had disconnected, checked and cleaned all the plug leads weeks ago. The external terminals on the dist cap looked fine. And I had been getting good spark and easy starting. But I had not removed and looked inside the cap. Clearly the terminals inside the dist cap were making poor connections, which got worse at higher revs and when warm. In some ways it was amazing that the engine had continued to run, albeit a bit roughly, for as long as it did.

New cap and rotor, and the car runs sweetly again.

I feel more than a little stupid, having suspected bad fuel pressure, dodgy sensors, electronic closed/open loop problems etc. Hence my memos above.

The moral of the story: as Friz and others wisely reminded us, the only sensible way is to go methodically and logically through the troubleshooting process. Making guesses, or making assumptions, can take you down some blind alleys.

So thanks again to all who posted advice. I just hope that this little saga will be helpful to others who might face a similar problem. Sometimes the fault is not as serious, or as obscure, as you might think!

br1anstorm