03 Evo dropping a cylinder at idle:

Hoping you folks can help. Car is an 03 Evo with zero mods and 105,000 miles on the odometer. It has always had the typical Evo idle lope but lately it has started to drop a cylinder randomly at idle. Sometimes it idles fine, other times it is on 2 or 3 cylinders. It doesn't stall, but it wants to. Driveability off of idle is perfect. No change in gas mileage or performance under cruise or boost - just when it idles.

Car has not thrown any codes and has been reflashed for the P0300.

I changed the plugs - no difference. Checked all the hoses for a vacuum leak with no luck. Tried two cans of fuel injector cleaner and a tank of 101 octane (not at the same time obviously) - no change. Although there was no reason for the compression to be off (always have run stock boost on the stock ECU) I checked anyways. No issues there. MAF has been cleaned (always ran stock OEM air filters).

I'm guessing that if the crank sensor was going I'd be having issues all over the rpm range and not just at idle. Ditto for the o2 sensor and MAF.

Any ideas?

Thanks in advance,
Fitz

 

How have you determined that cylinders are not firing?

 

The engine will lope unevenly... excessively unevenly. Throwing a timing light on each cylinder doesn't yield too much in regards to which cylinder though. When idling it's almost sinusoidal... a definite pattern to it but without being able to trace it to one cylinder.

I'm really surprised it hasn't thrown a code.

Thanks,
Fitz

 

Have you change the sparkplug wires.Maybe its a coil if the same 2 cylinders are failing

 

Do a compression test see if compression is ok. Make sure no cylinder is too far off from another.

 

I haven't changed the plug wires. That's next on the list. I guess I'm puzzled by the randomness of it and the fact that it only happens at idle. Cruise is fine. On boost is fine. Just idle. If the coil pack was failing I'd expect to see it under boost due to the higher cylinder pressures and definitely under cruise due to the leaner AFR.

Although there was no reason for the compression to be off (always have run stock boost on the stock ECU) I checked anyways. No issues there.


Thanks,
Fitz

 

leaking blow off valve?

 

At 105k have the plug wires ever been replaced.

 

Replace the plugwires and also reset the ecu (yank the power from the battery for a minute), see if that helps.

 

Once at 35k and again at 75k. OEM wires.

It's not leaking at idle. BOV is also OEM.

Sincerely appreciate all the suggestions folks.

Fitz

 

lower voltage at idle could be the difference between good performance off idle and issues at idle. Either plug wires or coils is where I'd look.

 

What about the battery?

 

Agreed. I'll pick up a set of plug wires tomorrow and price out the coil packs. Anyone have a vague idea of how much these go for?

No issues with the battery. Car continues to start prompty every time even with the extended winter we're seeing in the Northeast right now.

Thanks,
Fitz

 

Problem solved.

The timing belt was excessively loose due to a worn/tired hydraulic tensioner allowing the cams to be all over the spectrum. I discovered this when turning the motor over by hand. The exhaust camshaft jumped a tooth and then another.

I replaced the hydraulic tensioner, reset the belt, and I now have a rock steady idle.

FWIW, the tensioner had 101,000 miles on it and two re-compressions when it finally gave up.

Fitz

 

^Good you found that! So it jumped teeth just by hand-cranking? Any possiblity it jumped doing normal operation? If not, you're one of the lucky few!