84 2.5l Slight misfire and rough idle after warm (Page 2/2)
zostendorf JUN 05, 07:36 PM
Good feedback Patrick,

You'll be pleased to hear I've got a new fuel pump on the way. Includes a new hose as well:
https://www.rockauto.com/en...9006&pt=6256&jsn=425

I also order a 195 degree thermostat while I was at it.

Pump gets here Friday so I'll drop the tank over the weekend, take some pictures, and report back. You've been a huge help. Thanks again.
Patrick JUN 05, 08:20 PM

quote
Originally posted by zostendorf:

You'll be pleased to hear I've got a new fuel pump on the way. Includes a new hose as well.



I still wouldn't trust the supplied hose with that fuel pump for any long term durability. Dropping the gas tank is no fun... as you already know! This is what I use.


quote
Originally posted by Patrick:

It's also important that the proper submersible flexible fuel line is used inside the gas tank. I doubt the short piece of fuel line supplied with new fuel pumps is the proper stuff, as submersible fuel line is relatively expensive. Regular fuel line used inside the gas tank just rots.


[This message has been edited by Patrick (edited 06-05-2024).]

zostendorf JUN 09, 10:47 PM
Well, I received that OEM fuel pump and installed earlier today.

I bench tested the pump before installing to make sure it operated correctly. After getting it into the tank and tank back into the car, I fired it up. Nothing changed. The car still runs the same as before, intermittently misfiring every few seconds at idle. I want to reiterate that the misfire is not rhythmic, but is happening every 3-5 seconds. The car has a really great cold start, warms up at around 2200 RPM without any issue. Then once the car is warm the car ramps down to the idle RPM around 1000 RPM. Then the misfire starts. Still no codes.

The most confusing part of all of this is my fuel pressure gauge now reports 0 PSI. The needle does not move. I had not altered the configuration of the fuel pressure gauge in any way between the old and new fuel pump. What's strange is that the car is running the same. It idles with the misfire but never dies. It is getting fuel. I messed around with the fuel gauge a little to see if something was wrong but it still continued to report 0 PSI. Only conclusion I can draw is that prehaps the fuel pressure gauge got clogged somewhere or something. I don't really know. It's just my best guess. Unless I meticulously failed at installing the new fuel pump somehow.

Not sure how to move forward. I did the new fuel pump install by the book. Made sure everything was seated properly. Used a new hose that connects the pump to the end of the sending unit. Made sure all hose clamps were nice and tight. I made sure all 3 metal hoses on the sending unit were properly sealed and that their welds weren't cracked or broken anywhere.

Any advise is welcome. I'll try and upload a video tomorrow. Maybe someone can shed some light.
Patrick JUN 10, 01:36 AM

quote
Originally posted by zostendorf:

The most confusing part of all of this is my fuel pressure gauge now reports 0 PSI. The needle does not move.



Well yes, it is a little strange that it showed something previously and nothing now... but as mentioned earlier, a 140psi gauge just isn't the right tool IMO to be measuring a duke's fuel pressure (with any degree of accuracy).

[This message has been edited by Patrick (edited 06-10-2024).]

zostendorf JUN 10, 01:27 PM

quote
Originally posted by Patrick:

Well yes, it is a little strange that it showed something previously and nothing now... but as mentioned earlier, a 140psi gauge just isn't the right tool IMO to be measuring a duke's fuel pressure (with any degree of accuracy).




Thanks Patrick,

I'm going to run a compression test this week and I am also expecting my ALDL adapter to arrive this week. Maybe we will get more clues. I'm going to find a more acute fuel pressure gauge that should be more accurate. Hopefully I can sniff out the issue with those pieces of equipment.
zostendorf JUN 26, 09:23 PM
To anyone still following this thread, I have some decent updates:

Fuel Pressure - Got my hands on another new gauge ranged from 0-15PSI, and happy to report that my fuel pressures measures at 11 PSI while the car is running. Unfortunately, the fuel pressure drops to zero instantly a few seconds after the car is shut off. I'm not sure if this to be expected, or if the fuel system is intended to stay under pressure at all times. Like I said, it drops after a few seconds after shutting off the car. Stays under pressure enough time for me to get out of the driver seat and step back to the engine bay to actually see it drop.



Compression - Compress test results were fairly good. Cylinders 1, 2 and 4 are all measuring at 110-125. Cylinder 3 measures around 75-85. I ran a compress test across each cylinder several times. Cylinder 3 never came in over 100 PSI. Surely this must have something to do with the rough idle and misfire.

Spark - I installed some new premium spark plugs and I was pleasantly surprised. This is an item I had yet to replaced being that condition of the ones that were already in the car looked fairly new and clean. After replacing them the idle is smoother. Although it's still a bit rough. Probably going back to that compression on cylinder 3.

Anyway, thoughts welcome. Just wanted to give an update in case anyone comes across similar issues to mine.
Patrick JUN 26, 10:38 PM
That's a much better fuel pressure gauge to be using!


quote
Originally posted by zostendorf:

Unfortunately, the fuel pressure drops to zero instantly a few seconds after the car is shut off.



It's supposed to hold pressure. If it's the injector or diaphragm that's leaking (into the throttle body), that might also explain the rough idle, as it would be running too rich.


quote
Originally posted by Patrick:

...the fact that the fuel pressure drops immediately is an indicator that there's a problem somewhere. It's usually an internal leak in the tank, but it could also possibly be a leak in the fuel pressure regulator diaphragm.


[This message has been edited by Patrick (edited 06-26-2024).]

theogre JUN 27, 09:16 AM

quote
Originally posted by zostendorf:
...there is some moisture in the exhaust after I have not started the car for several days. this moist dissipates after the first 30-60 seconds. I assume a leaky head gasket may be the cause.

Maybe condensates w/ cold exhaust & cat...
Pull the spark plugs, keep them in order, look @ them carefully.
Is 1 plug like #4 is cleaner then others?
Yes? #4 cyl will get coolant suck in thru intake w/ bad gasket. Replace w/ Felpro MS90563. If others are clean, has way more problems.

Fuel Pressure https://www.fiero.nl/forum/Forum2/HTML/146121.html TBI Test Adaptor for Duke etc.
If leak down after shut down instantly, can be leaking "rubber" line @ pump, iffy pump, Iffy Regulator or Regulator seat on TBI.
You used Tape to seal the wrong adapter then "Boogers" can F' the regulator &/or the injector.

ECM scanner is good but often will not help finding your problem above.

Pull Tach Filter. See https://web.archive.org/web...t/~fierocave/hei.htm & Ground "Myth" notes link to that page.

------------------
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

[This message has been edited by theogre (edited 06-27-2024).]

Fiero-88 JUL 18, 12:19 AM
I have a extremely similar issue with my 88 for the past 2 years, just my issues got a lot worse over time, but I was able to get it back to just misfiring in park, mine had water in the gas causing it to stall. I replaced basically everything you had to no avail.
Anyway I had a aha moment with my car but haven't gotten a chance to see if it's what's wrong with it, so I figured I'd share my thoughts with you as well.
I was thinking it could be the break booster or a break booster vacuum line.
With mine it misfires when you let off the break only in park, and used to only misfire in park like yours, so I was thinking those two things could be related. Break booster could be causing low compression.
Gizmo0816 JUL 18, 12:09 PM

quote
Originally posted by zostendorf:

My stock 84 TB does not have a valve/port available to measure the fuel pressure, and I have been hesitant to insert an inline fuel gauge to avoid cutting into anything. I understand there is an inline fuel pressure gauge that threads into the line where the fuel filter goes, but I have had trouble finding one online.





I got a filter adapter I'll be using tomorrow, if it works I'll send you a link for it. I have a 88 2.5, but I imagine the filters are the same.

Just realized how old this thread is lol.

[This message has been edited by Gizmo0816 (edited 07-18-2024).]