Hello there, I have a new-to-me 2000 Chevy Express Sportsmobile that's giving me and the mechanics all sorts of fits. It had an occasional intermittent misfire when I bought it that got progressively worse on a test-trip of a 200 miles. I took it to a mechanic once the "service engine soon" light came on who determined the MAP sensor and MAF sensor were failed and he replaced them. Driving home, it continued to misfire. I took it back to the mechanic who over the course of several weeks checked connections and tested other possible problems but was not able to determine anything obvious. He reset the computer, it drove fine for about 100 miles when the misfiring started again. I took it to a more specific and closer in proximity Chevy mechanic who tried various known good parts from a similar van on my van but was not able to determine anything that would cause the code PO175, fuel running rich. He did pull apart a fuel line and determine the vacuum seal (I believe) was not working and replaced that. He also, under pressure from me, replaced the crankshaft sensor to try. "something" as I was now going on 1 month in the shop without solutions. This didn't solve the misfire or the codes. I took it to a 3rd mechanic, who upon advice from an out-of-area Chevy specialist replaced the 02 sensors. He also noticed a rub on one of the sparkplugs and recommended a tune-up, uncertain why this hadn't been done prior. He did a tuneup, re-set the computer and to no avail, the misfire returned. 6 weeks and $2000 deep into my "new" van, I thought about hanging it up and selling it but decided to replace the computer as the 2nd mechanic found one for $145. It drove fantastic, for 4 drives equalling 40 miles with the mechanic, 40 miles with me, an overnight trip of about 100 miles total, and then on testing it slightly further on a 250 mile trip--the misfire returned. The "service engine soon" light did not come on initially, but today on the final 50 miles, it was on and misfiring at regular intervals. I'm not sure what to do next, both local mechanics seemed stumped and frustrated as well, assuming it's a hard-to-find wiring issue, or something. Suggestions welcome....

 

A coil heats up and then fails?

Is it dropping a cylinder, do you think, or intermittent?

My suggestion is to start performing the work yourself, unless you are short on time and have plenty of $$.
You can sit down, leisurely inspect, for example, each coil, or have an assistant put a timing light on each wire.

Are there any cracks or major leaks in the exhaust manifolds?

 

What does the coolant temperature sensor say?

When they fail they tell the computer it's cold. When the computer thinks it's cold, it runs rich.

It could be an injector has dirt in it and sticks partly open. But I think that would turn up faster and not be affected by computer reset.

Has anyone tried to isolate the misfire to a particular cylinder? I know it's a bear to reach around in there but you could pull the plug wires off at the distributor one at a time.

The distributor has a plastic body, sometimes they break when the cap is removed. They can also wear and that causes problems.