My 3400 problems... going on for over a year...

[heavywoody]

Ok, this is going to be long winded, so consider yourself forewarned.

Engine: 3400 from 99 Grand Am, 22K miles, sat outside under a tarp for probably about 3-4 years before I got it. I tore it down to the block (leaving the lower end alone) and replaced every gasket/seal/o-ring possible before putting it in the Z26. I had the heads milled and pressure tested to make sure they were good and straight. I did not mess with the timing chaing. While the engine bay was empty, I replaced the steering rack, cleaned the engine bay up, replaced all the AC components under the dash (evaporator, the thermistor thing... I can't remember the name of it, and an L pipe leading to the front of the car), and cleaned up the heater box.

My problem is that I have a misfire on hot-restart. A BAD misfire; IIRC cylinders #4 and #5 had weak/no spark according to the spark tester. Sounds simple, right? Not so much. The car is dumping extra fuel in like crazy, and if the car is driven around while it is misfiring, the cat will glow red due to the extra unburnt fuel being dumped into the exhaust.

There have been so many parts tested/swapped/replaced that I'm just going to list them in easiest order. All sensors with the exception of the coolant sensor were purchased for a 95 w/ 3100 V6.

Coil packs - have been tested, swapped out, all are good.

Ignition control module - has been tested, swapped out, both checked out ok.

O2 sensor - was new, replaced with another new one, checked wiring, all is good.

Crank Sensor (rear) - has been tested, swapped w/ new, both checked out ok. Also checked wiring for shorts/melts/etc... wiring good.

Knock sensor - has been replaced. Suggested by Ben @ 60 Degree V6 when I picked up my heads from him.

Cam sensor - has been replaced, was replaced w/ new when engine was rebuilt.

Crank Sensor (@ crank) - was replaced when FFP UDP was installed.

Coolant sensor - new 3 pin style was bought and I converted the main wiring harness over to accept new harness when installing the 3400. I double checked the wiring many times, the wiring to the harness is good, and the wiring in the harness is good too.

Fuel injectors - using the ones from the 3400, these are NOT Multec II's... these are just like the ones I had in my old 3100. I sent them to Caspers, and had them ultrasonically cleansed and flow matched. They marked which ones go to which cylinders, and they were all put in the appropriate spots. FYI, I have swapped back in the original 3100 injectors, and those didn't help my problem.

Fuel Pressure Regulator - using Caspers Adjustable unit that was/is available on PFYC.com. I bought this back in 2001? Decided to use it now. Not sure if this could cause what problems I'm having or not, but it isn't leaking at all. FYI, adjusting the pressure in the rails does nothing for how the engine runs... which I find odd...

Fuel Pump - replaced on word from John Carter (JC); Bosch unit in place now, and is obviously functioning well.

Fuel Injector harness - tested each pinout for proper readings, everything checks out ok.

ECM - JC sent me a spare to try, and it didn't help anything.

This car is driving me batty...

Now, the reason I took the heads off and had them reworked was because a guy I worked with down here thought I should check the vacuum. It was the one thing I hadn't checked, and he said that if my vacuum was below ATM (14.7psi), then I probably had at least one stuck valve. So, I tested this theory out, and the car was between 15-20psi when it was running fine... and when it can like crap, it was around 12.5psi. I figured I had a couple stuck valves (in cylinders #4 and #5) so I had Ben @ 60 Degree V6 put in new SI valves, Comp Cams springs, new lockers/seals/etc and ported everything. Obviously the headwork didn't help solve the problem.

So right now, this is what I'm thinking, and HOPING will work. The coolant sensor is supposed to tell the computer what the temperature of the engine coolant is so that the ECM can compensate the A/F ratio (rich if cold, lean if hot). I don't think the sensor, for whatever reason, is sending the correct information to the ECM, so the ECM isn't leaning out the mixture when the engine is started while hot. I'm going to convert back to the style that came on the car to see if this helps. I don't understand why the new setup isn't working, but it's not.

The car will start up and run just fine when it's cold, and after it gets up to temp. It does run rich after warming up though... but it can basically be driven anywhere as long as it has enough gas and it doesn't stall. The second it shuts off/stalls after warming up, it will misfire on restart. If it cools down completely (8hrs or so), then it can be started up and driven again. I was driving it back and forth to work at my last job in Michigan, between the time I sold my Grand Prix and acquired the Scion.

Lots of **** to read, but I believe that's everything.

Update: I just got off the phone with my dad... he put the 2-pin sensor back in, and used the 94-95 3100 fuel injector harness with the correct wiring, trying my theory... and guess what.. IT DIDN'T WORK! Wonderful.

Does anyone know what else I can check? The car is up in MI, I'm located in NC... so time to work on the car is few and far between... my time (and money) is precious with this car. I just want to get it driveable!

Please help!

[rweatherford]

My thoughts were on the coolant sensor.  Perhaps there is moisture/corrosion at the ECM inputs?  Since this car sits a lot it will be difficult to figure out.

If you can get a scanner on it you might learn a lot while watching all the ECM parameters while it is running and then while it is acting up.

Good luck!

[docrodg]

Welcome to engine swapping.  This happens.  You could try and check the fuel return line and make sure it is not clogged.  

Otherwise it seems most of the bases are covered already off the top of my head.  Unless the block is warped, it sat outside for so long I can't understand why you would not replace the bearings and check it for level head surface altho you did mill the heads so only half of the mating surface was trued.

 Could have blown a gasket ring and be leaking fuel into the neighboring cylinder, that would cause a fuel dump into a cylinder that would burn in the exhaust.  

Otherwise it seems that whole fuel control loop has been verified for operation and so has the ignition system.  The only real controls for the fuel are the coolant sensor and the O2 sensor.  The rest are involved more in timing of the spark than fuel control.

[wicked-irocz]

I was thinking about to much fuel pressure also.  Do you have a gauge?

[1988GTU]

Here's my opinion.
The only reason it runs fine on (cold) start up is due to the fact it is in open loop.
In open loop, the fuel trim is ran off of default settings in the PCM.
Once in closed loop (certain temps required for this to take place) it then uses the sensors to become more accurate.  I would say oxygen sensor is faulty or ECM (wiring, corroded pins) has some issues.

[Cliff8928]

The oxygen sensor has already been changed. I think it really needs a scanner hooked up for a more thorough diagnosis.  Unfortunately practically NOBODY will touch the car because of the custom work done to it.  This thing runs INSANELY bad when it's warm, the description Geoff gave doesn't do it justice.

[docrodg]

jumper the thing into open loop when warm and see what happens.  IF it runs better then it must be a closed loop sensor or ECM.

[revenger]

I bet it would be a computer problem also. sounds like the only thing you havent tried.

[leadfoot93]

if the 3400 runs hotter because it's a larger motor than your stock coolin system was designed for and your using the same engine temp range in the ecm it could be trying to rich up the mixture to disipate heat.  Also slap a guage on the rails and check your pressure.