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I’m working on my uncles 94 s10 blazer with the 4.3 CPI. The truck has sat for a while and initially got it with him complaining of a hesitation issues. Instead I found it to be missing from idle on up. No codes to be read so I checked and Fixed a few vacuum leaks from bad hoses, replaced the map sensors as I found it was busted,did a compression test showing a worn but still runable compression on all cylinders with a few psi of each other. Passenger side bank plugs all showed a little lean but not bad. Plug wires all tested out good and looked fairly new. Installed new plugs and checked the cap and rotor which looked new as well. Getting spark to all wires. With the one side looking lean and a possible intermittent vacuum leak in the upper plenum gasket, I seperated the intake to see what I could find. Fuel lines were leaking in the intake and the spider assembly was a little gummed up so replaced all that and a new gasket. After this the constant miss was gone and it ran great from idle on up when in park or neutral, but once in gear it would immediately miss and backfire into the exhaust. Checked the timing and was just a couple degrees off so put it back to factory spec and still had the issue. Checked the distributor and found some slack that seemed a little much but not bad enough to worry about since I can’t track down any shim kits and didn’t seem bad enough to warrant the cost of a new distributor (only used one around had same amount of play). Cleaned up the distributor contacts and replaced the coil pickup (looked kind of burnt),Checked the ignition module and it tested bad. So a new module and cap and rotor (lifetime warranty so why not) later I still have the same issue and I’m kind of at a loss. Motor runs to the moon and back smooth out of gear and only does it when in gear so I don’t suspect the distributor play is an effect and I don’t see fuel pressure as an issue or else I would think it would miss when reved up out of gear. Will check pressure in the am though to be safe but if you could point me in the right direction I would really appreciate it. Also I did disconnect the distributor feed to the ecm before setting the timing and did check for the misfire with it disconnected but still had it.
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Just how much can you turn the dist rotor back and forth? There should be no play in it, what its doing with the play is under load its throwing of the total timing advance and could be the source of your problem if the dist gear or cam gear is worn that much.
First post, hi everyone.
Still you didn’t replace the plugs and wires. Above poster is correct there should not be slack in the distributor. However plugs and wires can look okay, but not be okay. Sitting for a long time they might be somewhat fouled by old gas. check that you have a good strong spark, no yellow sparks. They should be blue and be able to ar .
The movement in the distributor is upward not rotational and as far as the plugs and wires, the plugs aren’t fouled up and I get a good spark from all the wires and resistance reading. If I was losing spark or had weak spark wouldn’t I have the issue all the way around and not just in gear?
What only missing in gear tells me is a light misfire where you don’t feel it when the load is light. Putting it in gear Increases the load. Can you tell which cylinDer is missing? what is funny is I had a car recently with a miss only under load. Turned out to be the injector harness had a pinch but only pinched under load when the engine moved.
Been thinking about the truck and wonder what the compression readings where. Did you happen to do a wet test? With it backfiring and the miss, I am wondering about the condition of the valves. Also, did you happen to get a fuel pressure reading? A fuel volume test would be good to do also, it could have good enough pressure to start and run with out a load, but not enough volumn to handle a load. Also, I know you said the dist cap looked good, With the shaft of the dist having up and down play, take another good look at the rotor and cap, look for any of the tiniest cracks or carbon tracks. Have you checked the O2 sensor? Longshot, but is the catalytic convertor still on it?
Ok so tested the coil readings comparing it to a brand new unit and it measured out fine on resistance, fuel pressure is 53psi consistently and goes to 60 when revving it up. On all wires I can get a good solid blue spark across a good 1inch or more. I checked the timing putting it in gear and it does advance about 3 degrees just putting it in gear but not enough for me to think its the culprit. Vacuum is 15 hg at idle. A dry compression test (couldn’t get oil into the cylinders for a wet test) showed 115 psi across the board, varying only a couple psi. Like I said plugs weren’t fouled up and saw now carbon tracking on them (or cap and rotor but replaced them anyways since I had an extra set.) or cracks in the ceramic. Probably gonna grab a new set tomorrow for good measure but tonight I’m going to pull the egr valve of and block it off to see if I get any different results. The cat is still on the truck and as far as checking the O2 sensors I’m not sure how since I can’t look at any readings/ values with a scan tool. Also I’m not sure how to check the fuel volume
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