Intermittent rough idle, sluggish and popping in exhaust - FIXED!

[J Mac]

I've been battling this for a few months now. Been searching the internet high and low but can't beat it. 2004 GMC Yukon Denali 195k miles. There are currently no codes stored and no check engine light. There has been a P0300 code once but that has been cleared. Issue started slowly a few months back. The truck will sometimes have a rough idle where if feels like a cylinder drops and when it does it can be felt in the whole truck. Take off from a stop light and if it's idling rough, it'll take a few seconds to clear up and then it runs fine. Sometimes it'll pop through the exhaust. Most times it runs absolutely fine. Condition is worse when A/C is on.

Truck has MSD coil packs and wires, GM platinum plugs, long tube headers, custom exhaust with Magnaflow cats, 2-in 2-out Flowmaster 40 series muffler, Airaid Jr CAI. CP's, wires and plugs done less than 2 years ago. Wires have heat insulators and are pulled away from headers. New injectors from Wester's Garage just installed. New MAF sensor. TB has been cleaned several times. I've sprayed around the intake several times with carb cleaner to check for intake leak and that seems good. I took it to a mechanic yesterday that had a scanner that displays the sine waves of the sensors. He said the crank position sensor was very sporadic between 12 and 24 volts and the MAP sensor was also all over the place. His diagnosis was timing chain. Does that sound feasible? I hate to dive into that and still have the issue after replacing it!

I'm supposed to be pulling our camper for a trip Memorial Day weekend and can't until I get this straightened out.

Thanks!
Jason

 

[01ssreda4]

I dont believe a timing chain would cause the issues you describe. I would replace any suspect sensors.

 

[rockola1971]

I agree. A timing chain problem isnt usually random. Whats its loose as a goose it jumps a tooth and then the fun starts. Not uncommon for a crank sensor to cause sporadic misfortune and engine puking and sputtering. Cheap enough, replace that thing and see if it clears up.

 

[J Mac]

Yeah...that's my plan to replace the crank sensor and then MAP sensor if needed. I had the crank relearn done and it ran good for a day or so then some of the problems came back. The popping in the exhaust hasn't happened since but idle problem is there again.

Thanks!
Jason

 

[J Mac]

Still battling this...had the truck on the interstate for the first time in quite a while. Ran pretty good until at 70 for a few mins...then the flashing CEL and running rough. Slowed down to 65 and after a few minutes got much better and no flashing CEL. Same problem on my return trip on interstate, this time started under heavy acceleration on the on-ramp. Flashing CEL stopped after a min and ran okay.

So now truck is at shop, they replaced the crank position sensor but still show misfires on random cylinders at idle. Crank relearn was done after sensor install. Two mechanics checking it out when I stopped by and one said he heard the snapping sound of the plug wires arcing. The wires are MSD and about a year old if that. I could see that being the issue if only 1 or 2 cylinder consistently misfired...but the misfire are damn near on all cylinders after monitoring the scanner for a few minutes. This is driving me nuts...and broke! Any thoughts would be appreciated!

 

[J Mac]

I don't want to jinx myself but truck is running good now. When connected to scanner it still shows a few misfires on random cylinders but they cannot be felt or heard. Truck is idling nice and smooth. Mechanic did find that the coil pack connector on Cyl 1 had a bad connection. He cleaned it and made sure pins are in good...that seemed to take care of the real bad misfire and flashing CEL. Not sure if that connector or the crank position sensor were the main culprit...or both. Haven't had the opportunity to hit the interstate again yet but will this weekend. It's been running great for 3 days now and idling so smooth that I thought the engine died on me a couple times when letting off the gas! Fingers crossed!

 

[Danny3737]

Make sure all your plug wires are secure on the plugs.

 

[01ssreda4]

replace the plug wires, dont care how new they are. <<<if it gives you anymore problems

 

[rockola1971]

I totally agree. If one is bad...they all might as well be bad. Stick a new set on there!

 

[J Mac]

Finally got it figured out and not too happy with what caused the issue...MSD coil packs which are about a year and a half old. Mechanic had a parts truck at his shop with same engine. Took the coil packs off that truck and mine runs absolutely perfect now! And he has no idea how old the parts truck coil packs are. Not sure if they are OEM but seem to be...they all match. And of course MSD only warranties the coil packs for a year and that's if they are are installed by a certified MSD shop. Truck successfully pulled our camper to the mountains of WV with no hiccups at all. Happy it's finally fixed...not too happy I gotta spend about $400 on new AC/Delco coil packs.