98 Grand Prix GT (the non-supercharged 3800)

The engine will turn over, run for maybe a second, and then turn off. All my electric power is fine, ruling out the battery or alternator.

Sometimes, it will start, bump around from 500 rpm to 2000 rpm, and then sputter out.

Sometimes, it will start, and remain idling at 900 rpm (normal). I can then hit the gas, but as soon as I let up on the pedal, it will turn off.

Service engine soon light is on. It's an automatic transmission.

I suspect the mass airflow sensor. Your thoughts?

7/7/2008 12:47:32 PM

IAC valve

try starting it with just a bit (rest your foot on the pedal) of gas

7/7/2008 12:53:40 PM

I'm an auto newb. IAC?

7/7/2008 12:54:30 PM

idle air control valve

7/7/2008 12:54:55 PM

As long as it's above idle rpms, the engine keeps running. As soon as I let off, it (usually) crashes.

It's currently in the shop, but I figured I'd try for a TWW opinion to see if the diagnosis matches up.

7/7/2008 1:05:11 PM

Well goddamnit. The shop just called and said that it's not acting up and the engine light has cleared.

As SOON as I get home, I know it's going to act up again. Fuck.

7/7/2008 1:14:48 PM

IAC god dammit

[Edited on July 7, 2008 at 1:40 PM. Reason : and there should still be a history code they can view ]

[Edited on July 7, 2008 at 1:40 PM. Reason : spell]

7/7/2008 1:39:47 PM

Quote :

"idle air control valve"

cheap part, the few i've replaced have been easy as pie too. shouldn't take you more than a 1/2 hour to locate and replace

http://www.autozone.com/R,APP425062/vehicleId,2396102/initialAction,partProductDetail/store,428/partType,00207/shopping/partProductDetail.htm

http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c15280090e6b

[Edited on July 7, 2008 at 1:44 PM. Reason : part]

7/7/2008 1:40:43 PM

Well, before I took it from them I beat it up some, and when I got back to their parking lot, the light came back on, so I sprinted in and told them to get the reader.

As I suspected: Mass Airflow low frequency.

They said worst case is that the sensor must be replaced (I think that's like $150 or so) and best case is they can somehow tweak and fix it.

This same sensor went out on me two winters ago. Exact same symptoms.

7/7/2008 2:45:19 PM

you can unplug it and it'll switch to speed density mode and run fine for getting around btw

7/7/2008 2:52:11 PM

^ If that's the case, then if the sensor fails (as it has), shouldn't it automatically switch to speed density mode??

7/7/2008 3:28:39 PM

it would make sense for it to do that but i honestly do not know. i would guess it depends on how much of a "failure" occurs. if it's just getting bad data and doesn't realize it then obviously its going to keep using it--if it's far enough off that the computer knows it can't be right or isn't getting anything at all then i would assume it goes to speed density mode.

i'm not sure exactly what the low frequency code means but it going off/on would lead me to believe the MAF is still being used at some point (try unplugging it if it stalls on you again and see what happens)

7/7/2008 4:04:15 PM

If it's been slow failing, then maybe I'll see some performance boost after it's fixed!

My Grand Prix is an 11 second car, after all...

...11 seconds to 60

7/7/2008 4:15:21 PM

Quote :

"If that's the case, then if the sensor fails (as it has), shouldn't it automatically switch to speed density mode??"

its not like these things work one day, then don't the next.. they die over a period of time, and during that time give all kinds of screwy readings... since this is really the only way your ecu knows how much air is going into the engine, and, thus, how much fuel to inject, it can cause all sorts of strange problems.

7/7/2008 4:52:54 PM

Well, now the car is only sporadically acting up, so they "can't hone in on it."

I mean, if the code said "Mass Airflow low frequency," then either I've got a bad MAF sensor or I've actually got low air flow, right? So, assuming I have air flow, then completely replacing the sensor should fix it, right?

7/8/2008 2:11:50 PM

do you have an aftermarket air filter

7/8/2008 2:13:44 PM

Well, yeah, I mean it's been replaced many times.

[Edited on July 8, 2008 at 2:15 PM. Reason : ]

7/8/2008 2:14:38 PM

i mean like a rice boy filter like a k&n or something not a replacement oem one

7/8/2008 3:43:23 PM

Heck no. It's a vanilla pontiac, no upgrades or anything.

If the new MAF doesn't fix the problem I'm buying a new car. This is ridiculous.

[Edited on July 8, 2008 at 3:53 PM. Reason : ]

7/8/2008 3:50:23 PM

well cause i was about to say that the protective coating from aftermarket filters get on the MAF of gms and totally fuck them up. but this isnt the problem. good luck with the fix

7/8/2008 4:22:30 PM

you have low flow because the IAC is failing

7/8/2008 4:23:38 PM

I would have assumed IAC valve as well.

But clean the MAF with some CRC MAF cleaner and see if that makes a difference.

7/8/2008 4:53:53 PM

If the IAC were failing, shouldn't there be a code to come in saying "the IAC is messed up," rather than "the MAF is messed up?"

Replaced the MAF. Car is running fine, but we'll see. We'll see.......

7/8/2008 6:27:32 PM

not necessarily

7/8/2008 10:17:52 PM

i had an engine too lean code but the car was running fine

everyone and their mom said, oh thats the o2 sensor

[no]

cracked intake hose

7/8/2008 11:18:40 PM

[Edited on July 9, 2008 at 8:39 AM. Reason : doh]

7/9/2008 8:38:19 AM

HAHAHAHA

Low MAF freq came in again, two weeks out of the shop and no engine light until today.

So what's this I hear about just disconnecting the MAF and running without it in "speed density mode?" The guy at Autozone said it can't be done.

Again, 98 Grand Prix GT 3800 non-supercharged.

7/19/2008 2:30:23 PM

why don't you just try fixing it the right way

7/20/2008 11:19:23 AM

I would love to. And how would that be? It's not the MAF, after all....the current MAF is only two weeks old. And it's the third one in three years.

7/20/2008 12:49:19 PM

sounds like you have a vacuum leak

7/20/2008 12:57:28 PM

on the camaro i had once upon a time it did the same shit--i went through 2 in a few months and finally just lost interest and left it alone, thats why i mentioned the speedy density mode earlier.. the check engine light will still be on but at least it shouldn't stall

as far as the real problem..i'd like to ^, he knows his shit

7/20/2008 1:01:06 PM

Where can a vacuum leak occur? I'll re-attach the flexible hosing between the air filter and the engine (maybe I messed it up the last time I put in a new air filter two months ago). Other than that, anywhere in particular I can look?

7/20/2008 1:33:49 PM

Quote :

"I'll re-attach the flexible hosing between the air filter and the engine"

wait, is it disconnected?

what kind of filter is it?

7/20/2008 4:01:10 PM

No I mean I'll take it apart again and put it back on - just as I had to when I put the air filter in. I'm saying when I did put the filter in, I suppose it's possible I did not re-seat the hosing 100% proper.

And it's just a regular filter. Whitish and creased with a black rubber gasket.

7/20/2008 4:57:25 PM

whats the actual code number?

7/20/2008 5:42:10 PM