95 S10 4.3L W engine misfire

Getting p0300 multi cylinder miss code and p0420 cat code. Has slight miss at idle. When in heavy road spray in wet weather truck has severe miss and/or dies. After about 3 min engine will restart with bad miss. After a few minutes runs fine. Replaced IC and coil. Stopped idle miss for a day. Has more power and shifts better but still dies when following trailer truck in rain. Cap and rotor were done about 4 months ago. Fuel pressure 60 and steady. Engine is code W with VCM. Any ideas?

P0420 has to do with catalyst efficiency. P0300 is random multiple cylinder misfire. All of the basics must be good: AC Delco distributor cap, rotor, spark plugs and plug wires. Use silicone dielectric grease on ALL terminals, including inside the cap. All oxygen sensors should be replaced at 100,000 miles. Check fuel pressure with key on, engine off and cold, fuel pump running, must be 60 to 66psi. Must remain above 55psi for 3 to 5 minutes after the pump shuts off. Post your results.

Things got better last night. Son had truck on interstate in rain following truck in heavy spray and it started to die. He put his foot in it, it downshifted, stumbled and he headed for the exit ramp. Put it in neutral and it continued to stumble and then fired out the exhaust blowing the front cap of the muffler. Threw only the P0300 code. With muffler blown you can hear more frequent single misses. Starts fine, runs good if you couldn't hear the miss. The P0300 reset itself this morning meaning it isn't catching the misfires I hear in the exhaust. I normally don't work on electronic ignitions. I swapped to medium and heavy diesels in the late 80's. If this was a carb I would definately say ignition. But with the injectors how do you tell if the computer isn't firing the injector and thus the miss or you aren't getting spark? I'll do the cap, rotor wires routine, with AC Delco and see what happens. I think it is high tension also. What has me confused is the timing. At idle I consider it bouncing wildly all over the lot. 23 to 25 to 14 to 32 to 24 to 36 etc. Is this normal? Is this the computer trying to adjust the miss? HO2S 1 and 2 at idle are swinging also from a low of 120 to high of 820 and back and forth. This to me means lean then rich and back to lean and then to rich etc. Both fluctuate together with #2 being 50 to 150 lower than #1. Idle speed is 600 plus or minus 50 RPM. Fuel pressure chaeck fine at 60. Holds it when shut down. Short term fuel trim is also floating. Again computer resonding to miss? Thanks for the help!

Normal for timing to be all over. O2 sensors are responding to fuel mixture in the exhaust. Fuel mixture is constantly changeing so O2 sensors will change also. A slightly higher O2 reading would indicate the fuel is not being ignited, (random misfire) allowing unburned fuel to accumulate in the exhaust system. It finally ignited from the heat in the exhaust system, and blew the muffler out.

OK did plugs, cap, rotor and wires. All ACDelco. Problems above idle appear to be gone but idle miss has gotten worse. Gone from light miss to harder miss. No codes. Timing swings at idle have settled down alot. HO2S #2 has settled down alot at idle. Idle is now about 700. TPS is showing 0.4%. Double check ign. wires and all seem solidily connected. Can't test EGR with tool I have. Reading book on EGR it shouldn't open unless "off idle". Is 0.4% enough to be off idle? Can I disconnect connector and see if idle corrects? Any other way to check EGR? Or is this the wrong place to look?

.4% EGR will not operate, and should be closed. Make sure plug wires are in the correct place in the distributor cap. Passenger side of cap, wires front to rear are 2, 4, 6. Driver side is 3, 1, 5. Have you checked fuel pressure and leakdown rate?

Checked plugs, all in correct location and on tight. Did cold fuel pressure check. 60psi and holds. Since I was working on it and hadn't started it decided to chect IAT and ECT to see if reading correct. Outside air temp was 30, IAT read 30, ECT 32. Then with out starting engine watched HO2S heat up. Readings on both started about .450V. Let them warm up until they became steady without starting engine. #1 went to .085V and #2 to .015V. I would have expected them to be about equal. Are these readings OK and if not which one is off? Started engine and still had miss. Watched idle come down as coolant temp rose. When coolant temp got about 180 idle had settled to 650 +/- 25. Took some local trips, highway and local roads, ran fine but still had miss at idle. Then it threw the P0420 cat efficiency code again. Book says to check all VCM-A grounds. Does the VCM-A have a case ground or is it just the main harness grounds at the back of the heads? Also got freeze frame data on this one. Load 23.1% Coolant temp 189 ST Trim +1.6% LT Trim -1.6% MAP 22.1 RPM 1651 Speed 55. Any ideas where to head?

Excellent idea to check IAT and ECT before starting the engine. If they're not within a couple of degrees of each other, and ambient temperature, theres a problem. The ECM watches those two sensors along with MAP and TPS while in open loop operation. When the ECT reaches approximately 180F the VCM goes into closed loop and watches oxygen sensor signals to adjust fuel mixture.

When the VCM runs the catalyst monitor, fuel mixture is switched from lean to rich, then rich to lean, one time, very rapidly. The VCM watches the time it takes for the oxygen sensors to detect the change. If the reaction time is too quick, the catalyst is not doing its job and P0420 is set. The VCM relys heavily on oxygen sensor reaction time for this test, (and for fuel mixture adjustments). Oxygen sensors produce voltage from .1VDC (lean) up to .9VDC (rich) when the engine is running. If they're within that range, the VCM assumes they are ok and will not set a DTC. However, the VCM has no way of checkng oxygen sensor reaction time to make sure it's within specs, that is the responsibility of the technician. When oxygen sensors are exposed to the exhaust gasses, their reaction time slows, the longer they're exposed, the longer the reaction becomes. At approximately 100,000 miles the reaction time has slowed enough to noticeably affect fuel mileage, and the results of the catalyst monitor can be skewed. Enhanced scan tools can measure oxygen sensor reaction time to pinpoint a problem. If the sensors have more than 100,000 miles on them, it's best just to replace them.

The difference in O2 sensor voltage indicates a problem, they should remain close to each other and rise at the same rate. The VCM does not have a case ground, the grounds are on the cylinder heads. Make sure they're clean and tight.

Finally figured out how to tell in the wiring diagrams which connector has the VCM ground in it and what pin it is. Tested at 2.6 ohms from connector at VCM to battery ground terminal. Thanks for the explanation of the HO2S system. I'm going to check in the AM if the miss is there in open mode. I believe it is not. That would confirm the HO2S #1 was the problem. Either way it needs to be changed, has 150,000 miles on it. And fuel mileage has dropped in last 10,000 miles from 20 on the highway to 17.75.