Misfire only at low RPM / idle

[tekdeus]

My car was running great until the timing belt pulley went, at which point I had a shop in Nevada fix it for me. I changed the spark plugs right before it went into the shop. Since then, the engine has a small misfire pop/surge every few seconds at idle and at low RPM cruising speeds, causing the car to surge a bit while driving. At higher RPM's and full-throttle/boost, there is no problem, it's all smooth. (My car has Lex AFM, 550's, SAFC)

Today I changed the spark plug wires(some of which were cracked), but it made no difference. I'm wondering what I should check next? The mechanic said that my crankshaft pulley woodruff key slot was badly worn and would make it hard to set the timing. Would the timing have changed just from changing a timing belt? I could check it, but it may not be accurate if the woodruff key is worn like that.

If the car pops/surges every few seconds, does this suggest that the occasional combustion cycle is not getting spark, and when it eventually does, causes a pop/surge? Could it be an injector issue? Excess fuel?

 

[suprahero]

If it wasn't doing this before, I doubt it's an injector or fuel issue. I would definately recheck the timing and make sure he's got it set right. Good luck.

 

[metaphysico]

In a normal situation with all else aside if you get rough idle but goes away with throttle then plugs are bad. If it misses when rpms are up or loaded but not when idle its usualy wires.

Now that is just normal situation with no work or anything else to go off of. I would check to see how much your off on your timing mark, you will have to mark and pull off the crank pully bolt and see how much free play there is. When I find "BAD" keyways there only enough to throw out an engine 1-2 degrees, so you could just go forward and back a few degrees and see if it helps.

Also on overlooked thing is these are non-interference engines which means in the event of a t-belt failure piston to valve interference is UNLIKELY but it does happen to hit sometimes, just depends on engine speed durring failure. Unlikely but I would still do a leak down test.

But after scaring the hell out of you for a min, it is probably a vac leak somewhere. U need to see if it is running lean or rich if you have a multimeter you can check the 02 voltage at the test connector, if you have access to a lab scope even better.

 

[tekdeus]

If it wasn't doing this before, I doubt it's an injector or fuel issue. I would definately recheck the timing and make sure he's got it set right. Good luck.

You were right, the moron mechanic had my cam timing advanced by a tooth! I checked my base timing and I've been running 20-25 degrees! Lucky I didn't hurt my 7M, because I've been doing lots of 15psi pulls as usual. Maybe my knock sensors saved me.

I reset the timing belt tonight and the problem is now fixed!

 

[CyFi6]

Sounds like time for a new mechanic!

 

[tekdeus]

Sounds like time for a new mechanic!

I broke down on the way to Vegas; had no choice!

 

[suprahero]

Code 52 to the rescue................:biglaugh:

I'm glad nothing was hurt.