Stumble off idle - runs fine all other RPM

[Silber]

When I got my car back on the road it ran perfectly, but has developed something I am trying to troubleshoot.

When you try to accelerate from a stop it will sometimes stumble until it hits about 2000 RPMs and then goes fine. I think I smell unburned fuel when this happens, so I’m not suspecting a fueling problem. Also, if it were a fueling problem it would fall on its face at higher rpm’s which it does not. It Idles perfectly.

I cleaned the mass airflow sensor and that seemed to help for a while but it seems to be back.

I have a Durametric and I’m experiencing no codes or misfires.

Most of the Internet wisdom seems to say this kind of symptom is bad coil packs or a bad MAF, so I have ordered new of both. MAF will be here this week, coils next week. I am suspecting the MAF because it runs fine at Idle and above 2000 RPM, when i believe the MAF is not referenced. Problem seems to be worse when it’s moist out so that leads me to suspect the coils. These parts have over 100,000 miles on them so does not hurt to replace.

What I am looking for is other things to check until parts get here, and if these specific parts don’t fx it. what else could be causing this kind of seemingly intermittent and rpm specific symptom?

Regards,

Steve

[blue62]

a stumble at low RPM "could be" due to a vacuum leak at the intake.
A vacuum test gauge cost about 15-20 dollars.
If it was me I would buy one and do a proper vacuum test.

[Silber]

Little bit of follow up, thanks for the tip on the vacuum leak, did some research and it turns out you can check for a vacuum leak with Durametric by looking at MAF values.

At idle MAF should be 10 to 15. If you have a vacuum leak it will be lower. My MAF value is actually higher, 25 to 27 at idle spiking to 90 if any throttle movement.

So, it looks like the MAF sensor actually is bad. A vacuum leak would’ve showed a lower number.

MAF arrives tomorrow, we’ll see if that fixes it.

Silber

[Silber]

Read that if the car has a bad MAF sensor, it will run better without it hooked up. So, I Unhooked the MAF sensor, and drove the car on a couple of errands, drivability was vastly improved. I think that may be the culprit.

Siber

[blue62]

Originally Posted by Silber

Little bit of follow up, thanks for the tip on the vacuum leak, did some research and it turns out you can check for a vacuum leak with Durametric by looking at MAF values.

At idle MAF should be 10 to 15. If you have a vacuum leak it will be lower. My MAF value is actually higher, 25 to 27 at idle spiking to 90 if any throttle movement.

So, it looks like the MAF sensor actually is bad. A vacuum leak would’ve showed a lower number.

MAF arrives tomorrow, we’ll see if that fixes it.

Silber

A low MAF reading can come from a number of things. Yes low MAF readings may point to a vacuum leak but that is a very general indicator. Also a proper vacuum test can show you many other things besides a vacuum leak. such as early or late valve timing early or late ignition timing. poor compression. poor valve seating.( Valve leak by). muffler or CAT restriction.