I thought I would post my problem here since I have searched and cannot find the answer. First off, my car is a 1989 Firebird 2.8 manual. This is the problem:

Car starts, engine runs great. When cruising at a set speed (usually ~40 mph, but it happens at all speeds) the engine begins to stumble. It will do this for about 30 seconds then throw a code (the code being 45, a rich condition). After it throws the code, the car seems to correct itself and run right for a few minutes, then it starts over.

What I have done:

I decided to check the fuel pressure. When the key is on, engine off, I had about 42 psi. It would leak down to about 30 after 30 minutes or so. When I would start the engine, I noticed the pressure did not decrease. It would, in fact, increase slowly. When it hit about 50, the engine would stumble. If I pushed the throttle, the pressure would drop slowly, then begin rising again. There was no gas in the vacuum hose of the FPR.

So I replaced the FPR diaphragm and while I was at it checked the injectors. 4 of them had slow leaks . I bought some of the Ford F1ZE's and they look great, no leaks.

Before replacing the upper plenum, I decide to check the pressure. Now, it primes to 42, then quickly drops to 20 (within 2 minutes). If I clamp the return line, the pressure is steady at 42.

I went ahead and reassmbled the intake, etc. and I runs waaay worse. I can get above 30 mph and it is backfiring miserably. I rechecked timing, it was spot on.

My question is, is it possible that the FPR kit I bought is bad? I reinstalled it exactly as it was. Would that cause the poor running and backfiring?

Thanks for the help! I am sorry for the long post, but I don't want to drop another $45 on an FPR if it is not the problem.


-Matt

 

backfiring, loss of power, could be a vacuum leak, check your vacuum lines and make sure your gaskets are sealing. really common issue.

 

Thanks for the reply! Is there a good way to check for a leak? Pressure guage shows ~20 Hg. I don't hear any hissing. Could a vacuum leak cause the fuel pressure to drop after prime, then increase steadily when the engine is on?

 

Quick update on something I tried.

Here is what is happening:
I started the car, then jumpered the ALDL. The CEL was blinking fast, indicating closed loop. The fuel pressure was at a steady 42 psi. As soon as the car went open loop (the CEL was blinking slowly), the pressure rose to about 50 psi and naturally the engine was struggling. I noticed it would jump from opened to closed, and when it would jump to closed, the pressure would drop back to ~42 and it idle smoother. It was jumping back and forth closed to open about every ten seconds. Any ideas?

 

ya start the engine...take starter fluid spray it at spots you think might be leaking. If the motor revs up you know you have a leak there. Its an old school trick of checking for the o-so-common vacuum leaks that happen on older cars. Just be careful where you spray it. Dont spray it at your hot exhaust. You know what im getting at here.

 

Thanks, I'll try the starter fluid.

BTW, does the plenum need any RTV or other material, or is the gasket by itself fine. Chilton's doesn't say it needs it. I installed two new ones without. Also, does the EGR tube need RTV?

 

don't believe the egr tube needs rtv, the plenum gaskets don't need it either, but it definitely helps.

 

If the pressure holds with the return line pinched, but drops with it open, it's a dead giveaway that there is a problem with the FPR. There's something wrong with the parts or installation.

Also, the fast blinking SES light with the ALDL grounded indicates open loop, while slow blinkning is closed loop(O2 Feedback). The longer the light is on in closed loop, the richer the car is running, and vice versa. Once a code sets, the ECM defaults back to open loop. Hence the fast blinking you're getting.

 

I had a problem with fuel pressure dropping like crazy. Pinched the return line and it held. One would think FPR right?

Nope.

I had the whole fuel rail out with injectors sitting on the radiator to look for leaks. I tried 3 different FPR's (had one from the 3.4 stuff and another from somewhere), and none of them worked. Finally tried the 3.4 fuel rail with the same FPR's. Pressure held.

Somehow the sealing surface of the rail itself where the fpr pushes against it was bad. The 3.4 rail was different, so I grabbed another from the junkyard.

Also, if you use RTV on the intake, make sure it doesn't have silicone, it destroys o2 sensors. Or make sure it's sensor safe. (that's what I read anyway)

 

Thanks, I'll get a new FPR and try it! I'll update this if it gets fixed (I hate seeing open ended threads).

 

If it ends up being the fuel rail, does anyone know where I could pick one up? None of the junkyards around here have 2.8 f-bodies. Are there any other 2.8 GMs I could pick one off of?

 

So I tried another FPR (swapped first one for free) and it is doing the same thing. I don't know where I could get a fuel rail, It doesn't seem that the parts stores carry them.

A note about the FPR diaphragm. I installed it with a tiny o-ring around the fuel return hole in the FRP. With the o-ring installed, it held pressure. Clearly it is not getting a good seal. Everything looks good and clean, is there an improper way to install it? I tightened the top in the order the instructions said.

Thanks!

 

I noticed nobody is mentioning fuel pump.

 

That's because of the symptoms that are presenting. If the fuel pump check valve was bad then the pressure would still leak down when the return line is pinched off. If it were injectors, then you would be able to tell by pressurizing the system and seeing fuel drip out from the injectors. If it's the regulator, then it would only hold pressure while the return line is pinched off, which is the case here. Those are the only three places for the fuel to go.

 

I don't think it is the fuel pump because I am holding pressure when the return line is blocked. I do not understand why this thing is not getting a seal. I think the return hole must have a small ding or something in it that prevents the FPR from sealing it. It looks okay, so it must be small. There is nowhere else for the fuel to go. I hung the fuel rail above a pan, pinched the return line and ran the pressure to around 60, then let it sit. After an hour, there were no leaks from injectors (including the CSI), and the pressure was only about 2 lbs less. As soon as I removed the clamp from the return line, the pressure immediately plummeted to about 20, then about 20 seconds later it was at 0.

Would this problem cause the engine to run rich? I reassembled everything and now it is running too rich. I can't push the throttle past 2000 rpm without a backfire.

 

A prolem with a regulator can cause both lean and rich conditions. From our description though, I would venture to say that you are getting a lean backfire instead of a rich one.

 

A lean backfire usually occurs in the intake, correct? If so, that is exactly what the backfire is. It is definately not in the exhaust.

So we have determined that the FPR is leaking fuel back into the return, causing a lean condition, hence poor driving and backfires, etc.

My problem now is, why is the FPR not sealing the return line?

 

Update:

I carefully replaced the FRP and made sure it was PERFECTLY centered. Now it holds pressure! So I reassembled everything (again) and it seemed to run great at first! But after about 30 minutes of running, it started the same thing it did before replacing the FPR and injectors. I would start stuttering, fuel pressure would increase to about 50, then it would throw a code (45) and run fine. I assume it is going into open loop.

So if it is running fine in open loop, then terrible in closed loop, can I assume there is a sensor problem?

Also noticed this, when I changed the fuel filter, I relieved all of the pressure but the line was still pouring gas after I removed the old filter, as if I was draining the tank. Is that normal?

 

Maybe it's your oxygen sensor, but even if that's bad, you've still got a fuel pressure problem.

 

Thanks for the help! I was getting a code 34 and I checked out the MAF, it had a nasty crack on the engine side of it, which I'm sure made a nasty leak. I pulled it out and noticed some radiator fluid inside of it. A while back the heater control valve exploded and I think it may have cracked the MAF. Anyway, I went ahead and replaced the O2, even though it was only a few months old (I assumed the leaky injectors and bad FPR running rich over it wasn't great) and now it seems to run better.

Still, when it gets hot (around 190*) it starts to stumble (it is not as noticeable now) then throws a code 45, and everything is good until the next restart. Any ideas what that could be?

 

making progress at least. hmm, i'll keep thinking on it, there's something happening, i just can't call it out for what it is.

 

Thanks! I've got an ALDL cable coming so I can datalog it. Hopefully that can show me something.

 

Ok, here is something new. I read a tech artice on this site about setting minimum idle and thought I would check it out. When attempting this, I noticed the idle screw was completely extended. In fact, the head of the screw had been broken off. I got the needle noses and adjusted it waaay down, then proceded to adjust it up as necessary.

Now the car is running right. I guess the idle screw set too high caused this problem.

Thanks for everyone's help!

 

great to see you figured it out.

 

Idle screw....so that was doing all this. A simple idle screw?

The reason why I ask is because I'm having the same problem and I was about to go
buy a fuel pump and FPR...where would I find this idle screw? I'm not very tech
but I'm learning so please bear with me.

thank you...

 

The idle air screw is on the right side of the throttle body, above where the accelerator pedal links to. You can adjust it with a 20 torx. Be careful, though, those were not meant to be adjusted. My car's PO had obviously adjusted mine (the torx head was broken off). There is a very specific procedure to follow when adjusting it involving the IAC and TPS. Search around the board for the procedure.

Good luck!