IROC-DAVE

My 1988 Iroc-z has a 350 5.7 TPI motor with about 60,000 miles on a rebuilt motor. Mostly stock except for the underdrive pulley, and a little porting on the plenum, and Edelbrock headers.
The car runs great cold, until it gets warmed up, then backfires horribly through exhaust and looses power. I have checked the timing, replaced the distributor cap, rotor, and fuel filter.
I have been told it could be a failing fuel pump, but the fuel pressure never drops when I put a pressure gauge to the fuel rail, and put a load on the engine. From what I see, that can't be the issue.
I drive this car regularly to work and I don't want to loose the car.
Please HELP ?! Ideas ? Suggestions ?

m16a2r

Did you replace the distributor ignition module or whatever its called. Most likely it is your problem.

IROC-DAVE

Yes.

Black88Z

This is a common situation and i dont mean to sound like an @ss by saying this, im really actually being serious, but did you try a search? Theres a lot of threads similar to this and im sure theyd be very helpful.


But, ill still help of course. It sounds like you have checked the right things thus far, I would try checking your CTS and temp sender (I believe thats what its called..?) next. Also a good idea to check the MAF and MAT sensor (manifold air temperature I believe,in the bottom of the plenum. A real PITA to get to of course!)

Ses light on? Any codes?

I have to ask so we all know, what was the timing at? Did you check it with the est line unplugged? And plugged in? What was the fuel pressure at? When you snapped the throttle to WOT quickly what did it do? Did you unplug the FPR?

Is the FPR plugged in? Try kinking/pinching the line and watch what the fuel pressure does, it should spike I believe. If not this usually indicates a bad FPR.

ASE doc

Inside the distributor, above the ignition module is the pick up coil. It produces an magnetic pulse when teeth of the timing core and pole piece pass each other. The ICM converts this pulse into a digital signal that the ECM uses for fuel injector and EST ignition timing. The ICM also uses this pulse to fire the ignition coil below 400rpm. The pick up coil should test between 800-100 ohms with an ohmmeter. If not, it needs to be replaced. This requires disassembly of the distributor. Or, just replace the distributor with a quality remanufactured unit.

Another possibility is a heat related failure of any number of the fuel injectors. The should ohm test at no less than 12 ohms. Test them cold and then again hot. The resistance should be slightly higher hot. Suspect any than test lower hot.

One more thing that is a possible cause of this kind of problem is the MAF sensor or MAF relay. The MAF sensor signal circuit should read 1.6 volts at idle and 4.5 volts at WOT 3,500rpm.

IROC-DAVE

I haven't received any codes, so that hasn't become an open option. I will be testing tomorrow all my relays and pick up coil as suggested. Thank you for all of everyone's suggestions.

IROC-DAVE

I tested the fuel pressure, and I have 40 PSI with cold motor. When I warm up the engine, the pressure drops to around 30 to 35 PSI. I think I do have a failing fuel pump for sure, as I have checked the FPR and it was fine.

ASE doc

Be sure to test pressure both with vacuum to the regulator and without. With vacuum attached, look for the pressure to jump up 5 psi when you snap the throttle. Place the pressure gauge where you can see it while you drive and look for pressure to fall off under load. FP under heavy load should be at least 40 psi.

IROC-DAVE

I had, and the pressure dropped from 35 PSI to 30 PSI when the throttle was snapped.

ASE doc

Try pinching off the return line and repeat the test. If pressure still drops off, you need a fuel pump.

EDIT: If it's better with the return line pinched off, replace the regulator.

IROC-DAVE

Well, a new fuel pump, new fuel pressure regulator, new cap and rotor, new ecm, and NO CHANGE. I have visited 2 garages, and put about a full weeks worth of hours in sweat blood and tears.

I give up. She's going up for sale. I cannot take this anymore.

mmadden55

You have fuel injectors sticking open when they get warm. One maybe more. That's why your presur drops. Replace your injectors.

IROC-DAVE

I have put too much money into it, already. I have decided to move on.

Thank you everyone.

ASE doc

I'm sorry to hear of your decision to let the car go. Hopefully the next owner will have better luck figuring out the problem.

IROC-DAVE

That's another problem. I have found that it is going to be very difficult to sell this car, in it's present condition for under $1500.00 . I need at least that much to buy another car that is more dependable.
It is not an easy decision to let go of the IROC-Z. Trust me, I have been enjoying this car for a few years, and I am heart broken I cannot keep it, without spending everything I have to fix it.
Unless something miraculous occurs, and I figure out what the problem is without spending more money, I have to let it go.

ASE doc

Man oh man. I really want to help you but I understand your frustration and wanting to walk away. Myself, I have owned my beautiful Beasty for 15 years and am more in love with her now than ever. I know she's far from perfect but she is an awesome car.

I have had her down a few times. Some were short periods for minor repairs, others were for months. For instance the first major work over when I performed the chassis, suspension and original engine rework. I also do not drive this car in any kind of wet or otherwise inclement weather. For these times when she is tucked away safely in the the garage, I have a little beater that I bought for 1,000 and I don't give a s**t about. This way, I don't have to let myself get too frustrated if my Beasty has issues. I can take my time and just let her sit tight while I get her figured out.

As far as what' is going on with your car, from your latest test results I have to say that something is not right with your fuel pump. Either the new pump is defective, there is sediment in the tank that is clogging the pickup screen, there is an electrical issue affecting the pump's performance such as low voltage or a poor ground. Just need to dig into it again and see what you find. Try a dead head test on the pump to see if it will produce 100 psi. Then, perform a free flow test to see if it will pump 2 qts of fuel in 1 minute. Watch the flow when you start and compare it to the flow after one minute. Has it slowed down significantly? I have diagnosed some very weird fuel system problems. They all end up being something stupidly simple that we just overlook at first.

IROC-DAVE

FIXED !!
Turns out it was the coolant temperature sensor. Even the new one I had put in had been defective, so I purchased an AC Delco sensor, and it runs great now !

onthedownlow

iTrader: (4)

Now I would have NEVER guessed that could be the problem. Glad to hear you got it fixed, now you don't need to sell your car anymore.

IROC-DAVE

WOW that was short lived.

I was cruising down the highway and she completely shut down. Engine died, and it would not start again.
Seems I am destined not to keep this car......
I will just keep trying o fix her.

IROC-DAVE

Well now, that turned out to be a vacuum line from the fuel tank, so it seemed to have shut down, as if it was vapor lock ? I waited an hour or so, then after putting the line back together, it fired up and runs great again.