Cylinder 6 not firing when hot

We have a 1992 Pontiac Trans Sport with the Chevy 3.1 V6 TBI engine.

Engine was running poor because of a faulty Coolant Temperature sensor (rusted-out pins). Changed the sensor and cleaned-out rust from the socket and now it runs smooth when cold with decent acceleration.

Problem is that cylider #6 is not firing when engine is hot at operating temperature. Engine shakes lightly but performance remains good.

Distributor, rotor, spark plugs, spark plug wires and Oxygen Sensor were all changed 1 week ago with new ACDelco parts.

Any idea what to check next?

Many thanks!

Since there only one coil and all the rest "electrical" is new...I'd have said "the injector"...but this is a TBI so no joy.
DO you have to add coolant from time to time ?

Coolant level stays ok.

The engine runs smooth when cold but gets shaky at operating temperature.

Is there any change a bad coil (still on the old one) could cause this problem on just one cylinder?

Is there any change a bad coil ..could cause this problem on just one cylinder?
That one coil is good for all of bad for all....at least in my book.
..your problem is mind boggling...What are the signs that lead you to identify THAT cylinder as the culprit ? Did you check (change) the distributor cap (you said "distributor"..I just wanted to be sure) ? It happens that when a spark plug/wire develops a problem, an alternate path develops in the distributor cap..and it stays even after the plug/wire condition is corrected.
Also..I'd double check that the new wires have a nominal resistance (probably around 10,000 ohms) and they are fully inserted in the distributor cap.

Thank you LMP! Checked the wire (brand new) and spark plug and they work. Took it to an auto-shop and they said that the compression is around 2 atm on that cylinder (it should be at 7 minimum).

Since then three things happened:
1. we set the Timing to Service Manual specs - engine starts a lot easier and there's less misfires/hesitation
2. put a valve cleaner in the fuel and cleaned the IAC pintle - engine quit dying at idle but the hunting is still there
3. repaired the alternative LPG fuel system - engine keeps it's idle perfectly on LPG, hunting is gone

Today I will take it to a CNG/LPG for a tune-up and than make a few miles of city-driving to see how it goes.

I had forgotten your car used LPG....anyway, this low compression cylinder let one think of a failed valve ....or failed gasket...I hope neither of them but .a compression ratio of ..2 atmospheres does explain your misfire and must have an explanation....


keep us informed..

A few years ago, a friend of mine asked me to stop by with my scan gauge because his supercharged 3800 engine wasn't running right. I saw engine oil sprayed over the intake and front valve cover, and the oil dipstick was pulled out about 3 inches. I new immediately what happened. I pulled off the number 4 spark plug wire, and the engine ran the same.
He neglected to tell me that he was stuck in the snow, and he kept spinning the tires and putting the trans into drive followed by reverse. I knew that he was getting the compression from cylinder 4 into the oil pan. He detonated cylinder #4 which caused the piston turn to mush at the top. He was lucky that a Pontiac mechanic friend repaired it by boring it .010 over for about $900. He got off cheap. Not often do I hear of a misfire caused by low compression...good luck

Hmm, the things is that there's no significant oil loss (still at the same level for a few weeks now). The oil dip-stick is at it's place and there's no significant blow-by under the valve cover/oil pan.

We did the LPG tune-up and now it's running on LPG also. The engine pulls fine and idles good but sometimes it gets some kind of hiccups at idle after a high-speed drive/high revving. There could be also a vacuum problem since it's getting too much air - we had the air intake partially covered with duct tape and the engine ran better that way?!

Maybe the loss of compression is due to some valve sticking when hot or other cylinder-head problems... will scan again for sensor values and get a log for comparison.

I have added two OBD logs:
- one with a cold engine https://docs.google.com/open?id=0B_XMlHgNrAcoT3pCZDRoblZrc0k
- one with engine at operating temperature https://docs.google.com/open?id=0B_XMlHgNrAcoaEprTFRmbTlmTkU