Thread: Missfire at idle, shudder at 40mph low rpm load

Hey guys, having some issues with my '99 Grand Prix N/A.

At idle I hear/feel a missfire and at when I'm at a low RPM load on the engine, it'll buck, sometimes hard, and it'll do it repeatedly until I give it more gas to downshift. I'm assuming the problems are related.

I need to check the codes again and write them down, but I remember having 4 codes, two of which are the cat. I was reading symptoms of the cat, and it sounds a little more severe than the issues I'm having, like complete loss of power etc. Unless this is just the beginning.

I was also reading about EGR today and how that could be gunked up. Any good how tos, or is that pretty straight forward?

I've recently redone the LIM intake gasket and it was running beautifully, new wires/plugs, gaskets, oil/filter, but after about 1k on the new parts, it started acting up. Where to start? Well, besides the codes, I'll try to get those written down.

Thanks guys.

What type/brand of plugs did you install? Check to be sure you have no vacuum leaks anywhere. And a partially clogged cat can run fine when cold, then as it heats up it can choke the motor down. If you crawl under it and hit it you shouldn't hear anything rattling around in it. If you suspect a clogged cat you can also temp. remove the upstream O2 sensor and drive it for a short bit. Yes it will be loud and cause an exhaust leak but it usually relieves enough pressure to let the motor run properly if the cat is clogging.

Originally Posted by FordMan77

What type/brand of plugs did you install? Check to be sure you have no vacuum leaks anywhere. And a partially clogged cat can run fine when cold, then as it heats up it can choke the motor down. If you crawl under it and hit it you shouldn't hear anything rattling around in it. If you suspect a clogged cat you can also temp. remove the upstream O2 sensor and drive it for a short bit. Yes it will be loud and cause an exhaust leak but it usually relieves enough pressure to let the motor run properly if the cat is clogging.

OEM replacement wires with NGK TR55s I think were the plugs.

If that's the symptoms of a clogged cat, then I probably don't have a clogged cat. It doesn't matter if I've been driving 1 min or 50 it'll buck down the highway, any kind of slight incline while in OD or even an incline at low RPM at 20mph. Seems like any kind of low RPM at load gives it issues.

I'll get the codes checked today after work and write em down.

The cat codes didn't come back since I last checked, but the P0401 was there which is Insufficient EGR Flow

Read more at: http://www.obd-codes.com/p0401

Any good how-to's on how to clean it out?

Just remove the EGR valve and spray it out with brake cleaner. Also, take a look at this thread as it is often not the EGR valve itself.

http://www.grandprixforums.net/threa...ighlight=P0401

Originally Posted by FordMan77

Just remove the EGR valve and spray it out with brake cleaner. Also, take a look at this thread as it is often not the EGR valve itself.

http://www.grandprixforums.net/threa...ighlight=P0401

I removed the EGR valve and used a brush to clean the exhaust side down to the header, probably 8inches, then into the intake side about 6", cleaned the EGR valve itself best I could. No luck on the miss fire.

I understand from reading that thread that a MAP sensor could throw a code, but the MAP sensor can't necessarily cause a miss fire correct?

Perhaps I need to take the throttle body off and clean the intake where the EGR flows into?