Rough Idle/Choppy Acceleration

[joserw]

Just recently the car has a rather rough idle under 1000 RMP and sometimes goes down to about 600K. Not a violent shake, but noticable.

Under accelearation first gear is fine, but second and third are choppy and it almost sounds like the car is choking on something? Any ideas what this could be. Motor/tranny have 87K on 'em.

 

[trinity_gt]

What year?

What mods?

Any work done on the car recently the coincided with the onset of these symptoms?

Any MIL (check engine light)?

Any active or pending codes?

Sounds like an ignition problem. If it's 99-04 you might be looking at a COP. If its earlier you might have wire issues or possibly a bad coil. You may have contaminants (oil, coolant) in one or more plug wells. You may have alternator output problems (bad rectifier diode causing excessive AC ripple in the system).

A rough idle may also indicate an EGR flow issue (valve is not closing fully at idle), a vacuum leak or an IAC problem.

 

[joserw]

It's a 2001 GT, it has exhaust back from the headers, and CAI, Accufab TB & plenum.

No CEL codes and it has just recently started doing this. Plugs only have about 10k on them. The last repair I did was to replace the alternator (the battery light was flickering on and off).

 

[Night Shifter]

definitely sounds like a bad cop

also check for coolant leaking from the intake manifold..it may have cracked and the coolant could be seeping into your plug wells...probably not the problem but doesnt hurt to check

 

[joserw]

Coil pack wouldn't set off the CEL? So I guess I have to pull everyone out and check huh? There is a coolant leak but it's by the thermostat housing and was dripping down the front of the engine. I guess I should have the intake pulled anyway...

What about a fuel filter?

 

[wmburns]

Short answer is it sounds like the ignition is breaking down. Check for moisture in the spark plug wells. Note, if moisture in the spark plugs is/was causing misfire and allowed to continue, the COP boots could be damaged. Once damaged, the misfire will continue even after the moisture is removed. It is possible to buy new boots. Much cheaper than buying whole new COP's.

If there is an active coolant leak occuring, this is the source of the problem. The SOHC motor is very bad about collecting any moisture in the spark plug wells. The moisture will ground out the spark and misfire/loss of power is the result.

Check for vacuum leaks. Especially to/from the IAC bypass line. However, a weak cylinder (or weak over all ignition) can cause a slow idle.

Clean the MAF. Use only a product designed for the job.

As for options, run a power balance test. The problem is that few ppl have the ODB2 scanner that can actually run one. The dealer has a great engine analyzer that would most likely quickly nail it down.

So this leaves older methods such as trial and error. However, in this case, I think the cause is apparent by visual inspection.

 

[Night Shifter]

Coil pack wouldn't set off the CEL? So I guess I have to pull everyone out and check huh? There is a coolant leak but it's by the thermostat housing and was dripping down the front of the engine. I guess I should have the intake pulled anyway...

What about a fuel filter?

do you have an aluminum crossover on your manifold? sound like what you have explained is a cracked manifold...if the coolant is leaking you should definitely check your plugs and see if they have gotten wet

a lot of the times a bad cop wont cause a CEL...