99 K1500 Rough Below 2.5k Rpm

wrustry · September 1, 2008

I've got a '99 Suburban K1500 w/ 5.7. It had been running rough and getting steadily worse. Finally got to the point that it wouldnt starty and, while cranking, felt like it had slipped some teeth on the timing chain. I pulled the distributer cap and got my wife to crank it to verify the timing/cam assembly was intact. Found some oddness inside the distributer. the distributer studs were really built up w/ oxidation and the burns were about 5 degrees past the centerline and there was lots of light brown residue all over. I put a new cap and rotor on and it started right away but runs REALLY rough, but only below 2500 rpm. Above that it seems to run okay. I tried setting #1 at TDC and timing from scratch, same results. Runs rough below 2.5k. I'm thinking a crank position sensor but was hoping for some confirmation / hints / tips from forum members before proceeding.

Many thanks in advance.

minibike132 · September 1, 2008

The brown stuff is because of poor airflow in the cap area. Clean that crap up as much as you can and locate the screens. There are two tiny screens at the base on either side of the module. Take a pick and pull them out. What you explained about the marks sounds like there could be some slop in the distributor gear, If I was already in there taking out the screens I would probably mark the distributor position and pull it out to inspect the gear (make sure you have a new gasket before you pull it out). I'm not super confident you'll find anything there but the airflow thing is a problem and no matter what you find you will probably need a new cap and rotor in a year depending on your environment if you don't fix the airflow so why not spend a half hour checking out the gear while you are in there.

Edit:While the hood is open take some electrical contact cleaner and hose off the mass air flow sensor (pay attention to the thing in the middle that looks like a resistor. Also clean up the throttle plates. I use GM top engine cleaner spray, I have used alot of items and I think it's the best but it's not cheap. Alot of other guys use carb cleaner.

wrustry · September 2, 2008

Thanks for the input. I put a new cap and rotor on it this morning. When I found all the crap in there I figured it couldn't hurt. It had been sitting in the same place for two months not starting every time I tried, but when I changed the cap/rotor it started right away.

I thought it might be slop in the distributor assembly so I pulled it and checked it. No slop in the gear or shaft. I also timed it from scratch (#1 at TDC and rotor pointed at the #1 distributer contact) just as a double check, but that had no effect. Still only runs smoothly above 2500 rpm.

From the burns/buildup on the distributer posts it appears the coil is firing about 5 degrees *after* the distributor post. Since I'm left handed and dyslexic, holding the cap right side up, above me and looking up into it, I'd have to rotate the cap counter clockwise to get the burns/buildup lined up on center.

My only reference is mechanical Volkswagens and Scouts but aren't the burns on the distributer posts supposed to be about centered on the rotor axis? Since it only runs smoothly above 2500 rpm, could it be the crank and/or cam position sensor playing heck w/ the timing and causing it to fire too late when below 2500rpm? If it had a vacum advance it would be a no brainer to test!

minibike132 · September 2, 2008

I have to say that I'm having trouble picturing what you are describing but if what i'm seeing in my head is what you are talking about then that heavy ionization that happens from lack of air flow can describe it. The spark is going to track to wherever there is a circuit and that brown stuff = a circuit. The crank sensor is a possibility. The cam sensor will have no effect on driveability in this particular application (as well as most others but not all) the cam sensor is only used to identify what cylinder is missfiring if there is one.

wrustry · September 2, 2008

I'll see if I can post u a pic of the cap that came off it some time today.

wrustry · September 2, 2008

Picture attached.

Drawing a line from the center coil contact thru the center of the cylinder contacts, the coil appears to be firing a good deal later than expected, after the rotor has actually passed the contact.

I miss vacum advance!

minibike132 · September 3, 2008

You have your diagnosis of the cap wrong in my opinion. I think what you are seeing there is evidence of bad plugs and wires or other cylinder conditions causing higher than usual voltage necessary to fire the plugs. I don't think that looks like an engine firing at the wrong time it looks like the spark was finding a way back because of the cap breaking down due to that high spark. The only evidence I can show you to support that is if you look at this pic a,b and c support what you say on it's own but if you look at d you can see that the spark is not early there, at d it cannot be explained by early or late timing. The explaination for d is high voltage and it does explain the other marks as well. I would be willing to bet the reason for your original no start is that what you are seeing is worse under the plastic and that spark was finding it's way to adjacent cylinders.

Edit: sorry forgot to add the new pic.

wrustry · September 3, 2008

That wouldn't surprise me. My diagnoses are rarely correct. The plugs and wires probably have less than 3k miles on them.

The compression on the cylinders last summer were all good excepting #6 which was low but w/i specs. Oddly enough, OBD (when I've gone down to the parts store to borrow their reader) has shown a misfire on #6. Spraying ether around the intake didn't detect any leaks. That was one of the first things I checked cause:

History - a few years ago an intake gasket sprung a coolant leak. While repairing that I destroyed the the injectors by cleaning the bottom of the intake w/ carb cleaner spray. Finally took that to the dealer for repair cause I dang sure couldn't figure it out. They replaced the injector spider assembly and it ran great after that with only a small random miss that you could only really feel running down the highway.

Then last year it started running bad and I found a big crack (gaping hole actually) leading into the cat assembly that played all kinds of heck. I welded that up and replaced the O2 sensors finally just replaced the whole cat assembly this spring. Ran pretty good for a couple months then degraded. Replaced the fuel pump this summer as well.

I'll pull the ignition module and clean the air ports in the distributer body and the MAF as suggested (NOT w/ carb cleaner! That was a darned expensive lesson )

More news to follow! And many thanks for the input!

wrustry · September 7, 2008

So I checked the distributer vents, and cleaned the MAF w/ appropriate cleaner. Changed the plugs this morning. #3 and #6 were fouled and a real bugger to get out. After that the misfire that had been on #6 moved to #4 and it was still missing badly. I timed it from scratch again just to make sure and it was still missing so I went and got some Bosch spark plug wires to replace the ones I bought about a year ago and purchased the fancy scanner cause it's my birthday this week The new wires smoothed it out tremendously and it's no longer throwing any codes. Scanner data attached. Was hoping a more experienced eye could take a look and see if anything looks ugly (other than the coolant temp, I'm still fighting w/ the radiator as well.)I found a reference that the cam retard offset should be +/- 2 degrees and ignition advance at 10 degrees. The scanner I got doesn't seem to be able to tell me the offset (though I haven't finished reading the manual yet) and the ign ava is running around 24 degrees, which seems HUGE!