troubleshooting drivability problem on '92

[crazyd]

I've got a longstanding drivability problem in my '92 GTP that I think I've finally narrowed down enough to discuss it here. There are several things that are going on and they may all be separate, they may all be the same, or anywhere in between.

This is the 3rd engine I've had in this car since I bought it. The first one was so bad and ran so rough that it can't be used as a basis for comparison. The second one ran well for about six months and definitely exhibited this problem as well as the third.

The symptoms are:
- rpms dip when you tap the gas and it's cold, but goes away in closed loop
- lets out a harsh jerk when I let off the gas - all temps
- surges around 2000rpm, worse when cold but still there when hot
- stumbles on light steady throttle under load from 1500-2000rpm
- cruise control lets speed drop 1-2mph then jerks to engage, and surges at speeds below 65.

I replaced all gaskets when I built this engine to go back in this time, including the head gasket. I have been unable to find any vacuum leaks and have checked and rechecked vacuum routing multiple times including the check valve (which I also replaced) that sits between the cruise servo and reservoir. I have swapped TPS, IAC and O2. I have checked and re-gapped spark plugs. But what's common to all of this is the injectors, rail and regulator. I've been thinking about the regulator but haven't tried watching fuel pressure yet. This may be where I go next with this problem, but if anyone can think of anything else I've missed let me know.

[pocket-rocket]

I would get a fuel pressure gauge just to see what the fuel is doing. Also, your ECM is common along with the fuel system. Have you been getting any codes? Do you have a spare ECM or know someone with a 91-93 LQ1 car that would be willing to let you try their ECM in your car?

[crazyd]

There have been no SES lights so I haven't looked at codes, I'm pretty sure it's clear. As for trading ECM's with someone, not likely. 5-spd car.

[robertisaar]

As for trading ECM's with someone, not likely. 5-spd car.

any ECM from a 91-93 3.4DOHC or 93-94 3.1MPFI from a W is the same. just swap the PROMs and it will be fine.

[GTP091]

Your problems with the engine are a little similar to some issues I've had with my car awhile back. It's likely your problem is linked to one or two similar problems. I think the key here is to find the common cause among all of the issues your having. Not likely ignition related as no miss? I'd hazard a guess and say you have a small vacuum leak and a few minor problems with your fuel.
According to haynes and my GM service manual the stumbling could be:
-spark plugs fouled ( you've checked this)
-fuel system needs adjustment
-fuel filter clogged
-faulty spark plug wires (you would have a miss to go along with this)
-intake manifold air leak
the surging:
-intake air leak (him again? hmmm)
-fuel pump faulty
-loose injector harness
-bad ECM (suggested)

Both problems suggest the solution should be checking for intake leaks (you replaced intake gaskets but did you test your handiwork by spraying brake cleaner on gaskets and looking for RPM change?).
Both problems also suggest fuel delivery issues either with the filter, the fuel pump, or the injectors (harnesses on tight?). Like pocket rocket suggested test the fuel pressure, and change the fuel filter, it's very cheap, and have someone else who knows their way around cars look at your harnesses on the injectors and where the harness connects to the ECM near the MAP sensor the problem could be staring you in the face and a fresh set of eyes might spot it.
The last one that raised my brow the most was the cruise dipping and then playing catch up. This is definately a vacuum leak. My car did the exact same thing and acted weird when I let off the gas. When I closed the throttle the RPMs seemed to rise for a sec and then fall off slowly instead of falling quickly. When painting my car I lost the vacuum ball on a road test and plugged the line with a golf tee. Problem was the car didn;t have enough vac buffer without the ball, and was also leaking vacuum at the golf tee. The cruise did the same thing dipped and catch up, dip and catch up. I put the ball back on and sealed with silicone problem solved. I painted the ball of course; there's a thread of mine on here somewhere about the cruise/ball issue I had. Check for more vac leaks at brake booster, PCV valve, the cruise diaphram, the FPR and the vac ball. get a mechanics stethoscope to test for vac leaks. After fixing the ball 100% I found two more leaks further up the lines I only could have caught with the scope.

Mike

heres that post: http://60degreev6.com/forum/f100/vac...servoir-t41686

[crazyd]

Mike, thanks for the informative post. There are two more symptoms I forgot to mention:

- RPMs fall off too fast between shifts
- Car struggles to start after it has sat more than a half an hour or so, but 7 hours or more and it starts right up.

I've discovered my first problem in the fuel system with the FP gauge - the system isn't holding pressure. Soon as the car or the pump shuts off, the pressure bleeds immediately to 0. Is this the regulator or the pump? That will definitely take care of the second problem and who knows what else.

[crazyd]

Sprayed the engine down with carb cleaner while it was running and have no vac leaks. Vacuum test showed 18 in. at idle. Fuel pressure while pump is running is 38psi at idle and jumping to about 54 at 0 vac.

[GTP091]

- Car struggles to start after it has sat more than a half an hour or so, but 7 hours or more and it starts right up.

I believe this is called heatsoak the air in the intake heats up so much it is much less dense and harder to start initially which partially floods the engine making the problem worse. My brothers old beretta used to do this especially putting in clutch to turn off the highway. he would just hold the the throttle wide open and it would cycle new air faster and would start better. I can't remember what he did to fix it though.

haynes lists possible solutions as :
air filter clogged ( I hope you keep a clean filter )
fuel not reaching the fuel injection system ( low pressure? maybe fuel filter is clogged; once again)
Corroded battery connections ( this is an easy check which you should have done from the start )

Here's the repair flow chart from the GM service manual for my 91 your 93 should be the same. much more useful than haynes.

[pocket-rocket]

That's quite an interesting concept there. I would also check to see if you have the right fuel volume. A pump can give the right pressure, but not be pushing enough volume through the lines.

[crazyd]

The check valve that is supposed to hold the fuel pressure has failed. Question now is where that lives... fuel pump? Regulator? Elsewhere? The fuel pressure test also revealed that it takes an unusually long time for the pump to reach full pressure (~3 seconds). Either way I'm pretty sure I'm looking at a fuel pump problem.

[pocket-rocket]

Fuel pump, sadly Ever drop a gas tank?

[crazyd]

Only on a Fiero, which is pretty easy. Is it difficult on these cars?

[pocket-rocket]

It really shouldn't be too bad on any car but still not a fun job. How full is it? A transmission jack would be nice for stability of the tank when it's coming down. That or fab up a wide platform you can lift with a regular floor jack to keep it from falling over when you are bringing it down. Before you drop it I would check the shape of the tank straps to see if you need new ones before you take it off the road (if this is your only vehicle). If you do, they are probably an order item as well. If it comes with a new harness section, don't just crimp the wires. I usually solder and heat shrink just to give the ultimate weatherproof and strong connection.

[crazyd]

Update

I started looking into what it would take to replace the fuel pump, and there were issues. The biggest one is the special tool required to remove the pump assembly from the tank, which nobody has in stock around here and it would have to be special-order, so I couldn't return it when I'm done with it. Pumps are around a hundred bucks new, so put together it's not a cheap or easy repair. I gave it some thought and decided that the easiest way to fix this problem is with a $10 switch. I ran a wire from the lower dash through the firewall to the "fuel pump diagnostic wire" and made a fuel pump primer switch. Now I just kick that on before I start it and let it run for about 5 seconds to prime the system and purge the air, and it fires up on the first crank.

[pocket-rocket]

Nice thinking. Good idea using one of those safety switches with the cover that will kick off if you bump it with your leg too.