Random stalling with LX9 engine!! HELP!! [Archive] - GrandAmGT.com Forum


View Full Version : Random stalling with LX9 engine!! HELP!!

09-19-2009, 12:18 AM
im having an excellent time trying to figure out why my car is randomly stalling. I spent two hours checking all the threads trying to get answers.
I have done the LX9 3.5L V6 engine swap but i have put 6k miles on it without a problem and now the car has the following symptoms. So i don't want to blame the different engine as its running all 3400 sensors...


- Startup at idle, will run anywhere from 30 sec to multiple minutes then just shutoff and stall. RPM with drop from 600-700 to 400 and sometimes come back but usually dies

- Coming to a complete stop at a light will cause the RPMs to jump down and then car will stall unless you try to give it more gas.

- Driving down the road at crusing speeds will make it try to stall at random times. The RPMs will shoot down from 1500 to around 400 and try to cut but will usually come back and car only dies 1/2 the time. When the RPM shoot down the battery light also turns on for a split second then goes away like it did turn off.

- Notice this mainly happens on warm startups not cold and usually will take longer for the symptoms if the battery has been disconnected.

- Also runs very lean when it attempts to stall. Sometimes O2 sensor reads fine for a while but eventually gives a very lean condition and will try to stall or run rough temporarily

Replaced the following in attempt to fix this:

- Alternator
- Battery
- Idle air control valve
- Spark plugs
- Spark plug wires

Scanned the following sensors with a scan tool for correct signals:

- Throttle position sensor
- Mass air flow sensor
- Intake air temp sensor
- Engine coolant
- Engine RPM
- Manifold absolute pressure sensor
- Oxygen sensor

Checked the following, tested:

- 7x Crank sensor, resistance
- 24x Crank sensor, voltage
- Knock sensor, resistance
- Ignition coils, spark test
- PCV valve, not sticking
- Fuel pressure, 42 PSI at WOT
- Compression, cylinder-1 190psi, 3-200psi, 5-190psi, 2-175psi, 4-175psi, 6-190psi
- Vacuum gauge, runs steady at 22 inHg then will eventual erratically drop to 15 or less and either come back or stall.
- No blown fuses from all three fuse blocks

AND THE BEST PART IS NO CHECK ENGINE LIGHT! No lean code, no vacuum leak, no misfire!!

any ideas????

- Fuel pump was replaced in 2003 by dealer
- Fuel filter is about 4 years / 70,000

From reading the other threads the only ideas i haven't tried are:

- Disconnect O2 sensor / test
- Check EGR
- Security relearn
- Crank CASE relearn
- PCM connections
- Clogged CAT converter
- Replace fuel filter
- Relay?

It makes no sense to me, it has the symptoms of a misfire or a vacuum leak but no codes and will idle fine till it fells like stalling or has a quick erratic idle then comes back. I don't think its an individual cylinder causes this problem but i could be wrong. Wouldn't it point towards something effecting all cylinders like fuel delivery or electrical vs something with air flow since there is no leak yet i have a lean signal from the O2 sensor? IDK


09-19-2009, 08:14 AM
Hmm, doesn't sound like a simple fix since you've checked so much stuff already. For now I'll say two things stand out immediately. The fuel filter is definitely over due for replacement. I don't keep them longer than 3 years/ 36k miles. If you have 70k on it, it's time for a new one. Other thing is fuel pressure. Stock my car had 48psi at idle and 55psi at WOT. 42psi at WOT sounds very low to me, and if you are seeing a lean condition low fuel pressure could be the cause. Maybe the FPR has a bad diaphragm?

09-19-2009, 11:02 AM
i think im going to try the fuel filter today...

i also checked each individual cylinder with no condition change.

42PSI is low?? I have an adjustable fuel pressure regulator so i can turn it up, i thought factory at WOT was 40? I was running that for a couple years would that cause a condition now?

its really confusing cuz it will start up fine on a cold start and warm up, it seems to only want to cut on warm conditions. I thought the car mainly used the O2 sensors for those conditions but i may be wrong. After its warm sometimes i can start it and it will run for 5 mins, sometimes it will die after 30 secs.

Fuel pressure does hold when turning the car to ON. One thing i noticed is the needle on my pressure gauge does violently shake within a 2PSI range. Is that normal or could it be the cause? or is it just caused by the problem. What i've been reading from other pages is if it was the fuel pump it would just work or not, could it cause such a random shutoff?


Could it be PCM / BCM related?

I think the security shutoff is based off the BCM but i thought the security light would be blinking.

I also have no instrumental lights yet all the fuses are good. From what i've read it could be related to a bad dimmer module but is that also controlled by the BCM?

09-19-2009, 12:21 PM
would the fuel filter being dirty cause a difference between a cold and warm engine?

I checked my BCM, wires look good and i took it apart, nothing looked bad.

09-19-2009, 04:48 PM
Got it!! ignition control module was fried! Brought it to Oriellys for testing and passed no tests. Replaced it and drove 5 miles and idle for a long time with no issues, lets hope it was the answer. Good to know if anyone else has these symptoms!

09-19-2009, 04:57 PM
That's good you found it. I'd still replace the fuel filter of course and adjust the fuel pressure. What do your fuel trims look like? The car will still run if the pressure is low because once the O2 sensor is warmed up it will see the lean condition and add fuel to compensate, so your fuel trims would be way positive (it's going to have a much longer injector pulse trying to add fuel because of the low pressure). It would also affect WOT air/fuel ratio. But anyway, yes WOT fuel pressure is way higher than 40psi. The flow rate on the factory delphi injectors are rated at 22.5lb/hr at 55psi fuel pressure, and the factory regulator on 2000+ cars is 48psi idle and 55psi WOT.

09-20-2009, 06:52 PM
I wouldn't think WOT performance would be TOO greatly affected.. Sure you may get a small amount of spark knock, but the PCM goes back to speed density based fueling at WOT anyways. Usually running much more rich than stoic for a safety cushion.

But yes, definitely worth it upping the fuel pressure and replacing that fuel filter. My thought is a $20 filter is cheaper than a $400 fuel pump.

09-20-2009, 10:41 PM
I wouldn't think WOT performance would be TOO greatly affected.. Sure you may get a small amount of spark knock, but the PCM goes back to speed density based fueling at WOT anyways.

Not quite. The pcm never goes to speed density mode unless the MAF signal isn't present. Even at WOT it uses the MAF signal, not speed density. It goes from closed loop using the O2 sensor to open loop not using the o2 sensor at WOT. I think that's what you meant? But anyway, since it's doesn't use the O2 sensor at WOT and doesn't make fuel trim adjustments, not having the correct fuel pressure will definitely make a difference in the air/fuel ratio and could lead to detonation.

09-24-2009, 05:32 PM
just got back from vacation, drove the car for about 1.5k miles so yea it was definitely the ignition control module...

I had no idea pressure was 48PSI stock i thought is was 40! lol no idea where i got that from, oh well. Thats very good to know cuz i been running 42PSI WOT for years. I do have a A/R gauge and at WOT it would be about 1/2 green and 1/2 yellow but i had a vacuum leak involved with my intake, long story... I fixed the leak when i fixed the car and tonight i think im gonna crank up the fuel pressure and see what happens.

I have a PDA that can do 0-60 times, be interesting to see if it increases. With just this engine i raced a Grand Prix GTP about a month ago and he didn't pull on me til about 80 MPH who knows if i was even running correctly!

I also noticed my MPG shot way up after changing all these parts trying to fix the problem.
I went from an avg. combined 22.5 MPG to a strait hwy 29.5 MPG when i was getting about 25-26 hwy.

09-24-2009, 07:58 PM
Sweet! Always nice when you pick up some mpg as well as better performance. :)

09-26-2009, 12:00 PM
I did turn up my fuel pressure and my LTFT went from around 10% to less than 1% so that made a difference. But i can only get my fuel pressure up to 52PSI at WOT. I got the caspers adjustable fuel pressure regulator I bought like 5 years ago. Is that normal for it to only go that high?

09-26-2009, 01:20 PM
I don't think so. Stock normally goes up to 55psi.

10-29-2009, 01:25 AM
great now my adjustable fuel pressure regulator stopped adjusting and it appears to be stuck at WOT. can anyone give me a quick response will this kill my gas mileage?? doesn't seem to really effect my air/fuel ratio gauge.

10-29-2009, 10:13 PM
It won't help. The O2 sensor can adjust for it, but your trims are probably off, and you'll be rich under heavier throttle inputs until you hit WOT. You probably will lose some gas mileage from it and if left alone long enough it might burn out your O2 sensor and cat early. I'd try to fix it as soon as convenient.