October diagnostic challenge (SOLVED)!

[Jeremy airjer W]

This time I'm going to switch it up. I'm going to give you the vehicle and the symptom and let you ask the questions. Depending on the questions asked I may or may not have the info. If I do it is probably a good question, if I don't it isn't necessarily a bad question but it may not be relevant to this particular problem.

The vehicle is a 2006 Pontiac Grand Prix with a 3.8 liter.

The problem: The vehicle intermittently looses RPM and throttle response under moderate to heavy acceleration (typical entrance ramp acceleration would be the best comparison) when the RPMs reach approximately 4,000. After a few seconds the engine comes back to life and the vehicle drives normally. Occasionally the vehicle will stall after the above happens. It will then not restart until it sits for several minutes. It will then start and run normal all day long unless you try the moderate to hard accel and the RPMs get to the 4k mark.

The check engine light is not on prior, during, or after the symptom occurs.

Let's here the questions!

Question number;

1 - 124,000 miles, air filter like new

2 - Maroon

3 - 4 door, w/ A/C

4 - Catalytic converter is not plugged.

5 - Wiring, connection, and grounds are good.

6 - MAF sensor reading 4.5 g/s when it would crank and not start. The numbers where the same when it cranked and started. At idle The MAF sensor was reading 5.5 g/s. Since all the numbers seemed right I ruled it out.

7 - Plenty of spark out of all the coils during the no start condition.

8 - Fuel pressure held a steady 55 psi during the no start and the symptom.

9 - Catalytic Back pressure at 3k RPMs (power breaking the engine) about 1.5 pounds. Within spec. No rattling from the cat and no leaking noises from the exhaust.

10 - Not sure if the PRNDL switch was dropping to neutral during the cut out or stall. Don't think it would make a difference during the stall?

11 - Injectors all tested O.K. at 12.6 ohms +/- a couple tenths

12 - Turns over just fine or normal during the stall.

13 - Did not check for injector pulse but I was convinced I had a fuel metering issue.

14 - Had both crank signals

15 - Did not look at MAP during the stall but I did look at it in the shop. Key on engine off it read 4.96 volts. Key on engine running it varied between 2.4 and 2.6 volts.

16 - Wires and O2 sensor are fine.

17 - Crank and Cam sensors are good

18 - Coolant temp sensor reading correctly. Never dropped, spiked, or flaked out.

[huntnfish]

How many miles on the car? How old is the air filter?

[Scott K]

What color is the car?

[PurpleFloyd]

2 door or 4? A/C or no?

[Jeremy airjer W]

How many miles on the car? How old is the air filter?

124,000 miles and the air filter looks like new

[Jeremy airjer W]

What color is the car?

Maroon

And your wrong because i thought the same thing!

[Jeremy airjer W]

2 door or 4? A/C or no?

4 door, power everything, and A/C. The radio and climate control where off during the test drive.

[huntnfish]

Catalytic converter plugged or partially plugged?

[Jeremy airjer W]

Catalytic converter plugged or partially plugged?

Back pressure in the exhaust before the catalytic converter was about 1/4 psi to 1/2 psi at idle, well within spec. A plugged cat with drivability symptoms is usually at least 1 to 1.5 psi at idle. Good question!

[Walleyehooker]

Should produce a code but I had a bad TPS do similar things

[Jeremy airjer W]

Should produce a code but I had a bad TPS do similar things

Funny you mention that.

I'm pretty sure this was the first year for "drive by wire." This means that there is no mechanical linkage between the accelerator pedal and the throttle body. The accelerator pedal has an "APP" sensor, actualy two, APP1 and APP2. The throttle body is now called the TAC module (Throttle Actuator Control). The TAC module is now the throttle, IAC, and TPS all conveniently rolled into one package.

[CAMAN]

What station is on FM preset number 3?

Bad wire harness or loose plug that when subjected to high acceleration would overheat or move and lose contact?

[Jeremy airjer W]

What station is on FM preset number 3?

Bad wire harness or loose plug that when subjected to high acceleration would overheat or move and lose contact?

Don't know, the radio was off

Wiring is good, connectors are good.

[gquist]

What does the MAF read during the cut out?

[Jeremy airjer W]

What does the MAF read during the cut out?

Excellent question!

The MAF sensor was reading 4.5 g/s (roughly) while cranking when the vehicle would not start. This was the same as when the vehicle would start. At idle it was reading 5.5 g/s (roughly). Since the results where the same for when it started and when it did not start I ruled that out.

[bucketmouth64]

I'm no motor head, but I'll say ignition module? Had somewhat a similar problem on an old Nissan Stanza.

[gquist]

How about fuel pressure, while running at idle and during cut out condition?

[Jeremy airjer W]

I'm no motor head, but I'll say ignition module? Had somewhat a similar problem on an old Nissan Stanza.

Good question!

On the second test drive I brought along a spark tester. When the vehicle stalled the first thing I did was check for spark. There was plenty of it out of all the coils.

[Jeremy airjer W]

How about fuel pressure, while running at idle and during cut out condition?

Good question!

Fuel pressure held steady at 55 psi the hole time.

[gquist]

I know you listed the exhaust back pressure at when the car was in the shop, did you test it during the cut out and stall condition?

Does the transmission gear position sensor read "drive" when in drive? (rev limiter in neutral)

[Jeremy airjer W]

I know you listed the exhaust back pressure at when the car was in the shop, did you test it during the cut out and stall condition?

[gquist]

Yea, rev limiter wouldn't make it stall, Map, tps, coolant temp and crank and cam sensors all a possibility, Injector resistance ok? This is where the scope comes out, never seen a oxygen sensor cause a no-start after a stall. Does it turn over normal after the stall? Has compression at that point, (lifters pumping up for some odd reason)

[Jeremy airjer W]

Injector resistance ok?

[jmd1]

Gas tank always full or half at least? Had a ford CV that when the fuel level got low, the heat from the return fuel would swell the pump. It would start off great and run good but then it would stumble and if you shut it down for a little while it would cool enough to start again.

[Scott K]

OK, let me throw in a couple questions, you said you have spark, how about injector noid? Have you tested the CKP, and cmp sensors?

When this thing stalls, are there any sensors that dont read correctly, such as Map, APP.

The 4000 rpm thing gets me, that is about when the rpm limiter kicks in when they get a bad ckp.

One more thing, I remember throwing a fuse box in one of these for a no start, because it would lose power to something, I dont remember what. I know, that isnt even a question.

It wouldnt be the bad wires, shorted O2 sensor wires by the fire wall that these had issues with?

Ok, enough of the stabs in the dark.