Trouble starting, then very rough idle and backfires (Page 1/4)
WingsGT FEB 11, 08:42 PM
Hello! Trying here for help as I've run into something that is a little out of my comfort zone for diagnosing. Last time I tried running it there was a very scary sounding backfire so I'd like some advice before getting back to it.

I have an '86 GT and one morning it suddenly would not start. I checked fuel pressure and spark, they are fine. Timing was at 10 degrees based on timing mark. I don't think the balancer has spun based off checking TDC with a screwdriver but I can't be 100% sure because I wasn't measuring that precisely. It will start with some throttle, but runs rougher than it ever has before and backfires dramatically.

I'm not a professional mechanic and just have a little experience with my own vehicles, so I'm feeling a lot of uncertainty trying to figure out what might be happening and what I need to do to fix it. Could it be fuel injectors? Cold start injector maybe? I bought new injectors so I may just go ahead with switching those, but it would be nice to know if that has a chance of fixing this or if there's something else I need to look at. I live in Florida so I am also considering deleting the cold start injector. If the CSI was leaking would could that cause this? Does that make sense that it would need the throttle open a bit to start with a leaking injector? Or could this be something else?

Thank you for any help!!
Patrick FEB 11, 08:58 PM

quote
Originally posted by WingsGT:

I have an '86 GT and one morning it suddenly would not start. I checked fuel pressure and spark, they are fine. Timing was at 10 degrees based on timing mark. I don't think the balancer has spun based off checking TDC with a screwdriver but I can't be 100% sure because I wasn't measuring that precisely. It will start with some throttle, but runs rougher than it ever has before and backfires dramatically.



What does "fine" mean? Keep in mind we have no idea of your ability to determine what is or isn't "fine". What is the fuel pressure while the engine is running? How quickly does the fuel pressure drop when the engine is turned off? Does the spark snap across a 1/4" gap to ground? Was the ALDL jumper in place when the ignition timing was set?

The ignition control module is a common failure point, but sometimes the issue is simply corroded connections from the pickup coil and/or the ignition coil to the ICM.

The issue could also be something as simple as a fouled spark plug.

Another simple thing to check is to make sure both injector fuses are good.

[This message has been edited by Patrick (edited 02-11-2024).]

WingsGT FEB 11, 10:03 PM

quote
Originally posted by Patrick:

What does "fine" mean? Keep in mind we have no idea of your ability to determine what is or isn't "fine". What is the fuel pressure while the engine is running? How quickly does the fuel pressure drop when the engine is turned off? Does the spark snap across a 1/4" gap to ground? Was the ALDL jumper in place when the ignition timing was set?




Thank you Patrick, you're right I could have checked those better. I just figured I knew it had some fuel and some spark. I did not check fuel pressure while running because I was too sketched out by the backfires once I got it going, but it was 45 psi with ignition on and held there after I turned it off. The jumper was in place when I set timing. I will check spark with a 1/4" gap tomorrow, along with the injector fuses and anything else you guys think I should look at. About a year ago I installed a new DELPHI ignition control module, NGK spark plugs gapped to 0.045, new spark plug wires and a Summit 40kv coil. I realize now it might have been better to use ACDELCO where I could. I doubt the spark plugs are fouled though because they are so new and the problem happened very suddenly, but maybe there was a sudden failure with those or something else in the ignition system. I have another ICM I could try.

This literally happened overnight, I had just got it running decently after sourcing a replacement vehicle speed sensor and letting it calibrate the idle air control valve again, which I think was the source of the problems that had originally led me to replacing all those ignition components. I drove it regularly for about a week until that morning when it wouldn't start.
1985 Fiero GT FEB 11, 10:05 PM
Patrick's suggestions are good, especially the aldl jumper, another thing to maybe check with a voltmeter is the TPS sensor, when that goes bad, it can confuse the computer into giving the engine the wrong a/f ratio, especially when starting, as there's no vacuum readings. When my TPS sensor went bad, it ran very rough, jumpy idle around 2000+rpm, and if I got on the gas too fast, it would backfire. The TPS is something that can be good one day, and either completely or partially fail the next.
Patrick FEB 11, 10:09 PM

A scanner or WinALDL would tell you much more, but at least check for Trouble Codes.
armos FEB 12, 04:07 AM
Also check for broken connectors in the engine compartment. The clips are frequently broken unless they've been replaced. Make sure they're all in good contact.
There's a connector plugged into the ICM that pretty much refuses to stay in place if the clip is broken, and it can cause these symptoms when it gets flaky.
There's also a short harness from the ignition coil to the ICM that is notorious for going bad due to heat exposure.

Listen for vacuum leaks - although on my car I've noticed the IAC is loud at cold start and sounds like a vacuum leak until it closes.

I think the CSI would have to be leaking pretty badly for it to cause these symptoms. But maybe it's been leaking before and this is just the point when it got bad enough to be noticeable.
The CSI isn't critical, but I tried unplugging it on my car and it took a lot longer to start even in warm weather.
Another source of unmetered fuel could be the fuel pressure regulator. It's possible for them to leak fuel into the vacuum hose that attaches from the regulator to the manifold. If you prime the fuel pump and then put suction on that hose, fuel should *not* come out.

Bad injectors are possible, but that would just be a guess. If one of the injectors have electrically shorted then it would disable the whole bank of 3, so then it would be running on 3 cylinders. It can run on 3 cyls with some throttle but I don't think it should backfire. A simpler cause of that scenario is a blown fuse as Patrick referred to.

Since the fuel pressure held when you shut the key off, it sounds like you do not have a leaking injector.
I'm not sure if this rules out a leaking fuel pressure regulator though. Without the engine running there's no vacuum, so maybe then it wouldn't leak the same as when running.

It could be an ignition issue, or a bad sensor reading causing the ECM to call for way too much fuel, leading to enough unburned fuel in the exhaust that it backfires.
Backfiring out the exhaust suggests it's either running rich or bad ignition, not lean. But any stored trouble codes could give more insight.
theogre FEB 12, 03:25 PM
Before buying parts...
Clean/fix all grounds & + wires in engine bay. Iffy wiring causes many problems. use silicon or permatex brake grease.
See https://web.archive.org/web...ierocave/ecmwire.htm & rest of section.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

WingsGT FEB 13, 01:45 PM
Fuel injector fuses are not blown. Spark at 1/4 inch was bright blue for the most part, immediately after starting to crank I saw a little duller color but I could have grounded my wire better probably.

Checked TPS, I've got 0.35V closed and 4.56V at open throttle.

The clip on the connector from the ICM to the coil is broken, but it is fully seated and secured with Tesa 51036 tape which seems to hold it very well, I can't wiggle it at all without pulling that tape off. The only other broken clip I know about is the MAP sensor, but it's been staying put on its own so far. I will try to look at the other connectors.

I will try testing the fuel pressure regulator with suction. I have a new diaphragm as well as new injectors, but I wanted to figure out more about what is happening before I take that all apart. And I do not have a replacement for the CSI. I don't really know what a leaking/failed injector looks like, but the seals must be good because it's not dumping on the intake when pressurized. Was thinking they could fail internally and lose their metering ability, dumping too much fuel when activated. But thinking about it after learning more about them, the metering comes from the PWM signal. Could the injector actuators get sticky and stay open longer than they're supposed to?

When I bought it it had vacuum leaks, of course they affected the idle but nowhere near this much. It is scary to run it right now and hard to hear vacuum over the ridiculously rough idle, so I'm really hoping that isn't the problem.

When it was running well, it still used a lot more fuel than I believe it should have. Something like 10-15 mpg, and the exhaust smelled a bit like gas. Maybe the CSI was leaking that whole time and recently got worse? But if it is always open, how does it hold fuel pressure? I will pull a spark plug or two (maybe all three rear ones, hate reaching the front) and see if I can confirm if it's been running rich for some time.

I'm also not sure it backfired in the exhaust, I'm worried it might have been intake. I was standing beside the car when I heard a very loud bang from the engine compartment, not so much from the tailpipe. But an intake backfire is not possible unless the timing is way off right? I know the balancer can spin, but mine isn't loose enough to move by hand and it was working with the same timing for several months. Whatever is causing this is something that can happen suddenly overnight. It went from driving decently and had a normal idle when I parked it one evening, to suddenly and consistently behaving the way it is now the next morning. Checking all connections and grounds is a good idea and I definitely will do that as I work on the car more. I've checked and cleaned a lot of them but not everything. But though I could be wrong, I feel like bad connections would cause a more intermittent issue and not the night and day difference I'm experiencing here.
Patrick FEB 13, 04:05 PM

quote
Originally posted by WingsGT:

When it was running well, it still used a lot more fuel than I believe it should have. Something like 10-15 mpg, and the exhaust smelled a bit like gas. I will pull a spark plug or two (maybe all three rear ones, hate reaching the front) and see if I can confirm if it's been running rich for some time.



With those mileage figures, I wouldn't be surprised if your plugs (one or more) are all fouled due to an excessively rich mixture. How old is your O2 sensor?

If your O2 sensor is okay, I'd then suspect the Coolant Temperature Sensor and/or the Manifold Air Temperature sensor is telling your ECM that the temperature is -40°. This is where a scanner or software like WinALDL really comes in handy.

[This message has been edited by Patrick (edited 02-13-2024).]

WingsGT FEB 13, 10:48 PM
I just pulled the spark plugs from cylinders 1, 3, and 5.Getting a bit fouled for sure, and 5 is much worse than the others. Image imbed didn't work but here's a link. The picture with flash makes it look a little cleaner than it does in person.
image

Without a lot of frame of reference, it is still worse than I was expecting. Though I did make a lot of short trips in the time it was running. Would this level of fouling be an immediate issue? Can still see the white in the insulator tip so it's not completely coated. Could this cause a problem to show up as suddenly as it did?

As far as the sensors... well, I replaced all of those already. I know now it's kind of dumb to just throw parts at it and new stuff isn't guaranteed to be good. When I got it, it had a radiator leak so when I replaced the rad I also got a new coolant sensor and thermostat. I later replaced the O2 sensor and intake air temp sensor hoping they would help with the fuel economy, but they made no difference. The O2 sensor is Bosch, so I hope that's a good one. The other two were cheapo. Maybe one of those has failed.

I'm going to try to get WinALDL. I have no idea why I did this, but I pulled the instrument cluster out to replace bulbs and swap out one of the gauges after this happened without checking the codes. I have it back together now and will reinstall tomorrow and find out what is stored.

[This message has been edited by WingsGT (edited 02-13-2024).]