[FIXED] 94 Sonoma 4.3W no power

Hello all. I could really use some help. I am about stuck. Truck all of the sudden fluttered and blew the muffler apart and hasnt run right since. Idles and revs up in shop perfect. As soon as you put it under load it falls on its,face like an ignition problem. Almost like the spark starts randomly firing or breaks down. Seems like an ignition issue. Here is what I have done so far.
replaced tank and pump fp good
AC cap ,rotor,wires
150LB comp. all cylinders.
EGR Ok.
No Cat,new muffler
New O2 sensor
ECT good
spark jumps approx 1/2 inch but looked a bit orange. Replaced coil, still looks the same. Grounded one cylinder at a time with test light. All cylinders contribute equally at idle.spark still appears a bit orange. Ig mod.??? ECM??? HELP !!!!!

have you checked back pressure? and fuel delivery?

Hey there kvnber4, we're happy to help!

The muffler blowing apart really says spark timing to me. Do you have access to a timing light? I'd be interested to know if the spark advances correctly while the engine is power braked (you'll need an assistant for this). Seeing the spark timing jumping around, or seeing the light stop flashing, would be a great indication of a spark control problem.

These distributors are known for cracked magnets AND tone wheels slipping off the distributor shaft, both of which can cause symptoms like this. If you don't have access to a timing light, you could try using an in-line spark tester on the coil wire to watch the spark as you're accelerating. I use this style of tester regularly, works perfectly:



Seeing the spark 'break up' or lose strength would be a big red flag for the ignition system.

Cheryl asked about fuel delivery, and I'm wondering the same. I see that this got a fuel pump and tank, but I'd still like to know the actual fuel pressure reading at idle and under load. Memory says it needs to be at least 55-60 PSI running.

Fuel pressure is right on the money. I used that exact spark tool and the spark is not blue. It will jump a good 1/2" but it is a bit orange. When I rev it up the spark remains the same. It sure feels like an ignition issue. I really appreciate the help from all. I will keep at it. Thanks!!!

another thing that can cause the muffler to blow apart is firing order on the distributor. seen it on an old beater dakota

kvnber4 wrote: Fuel pressure is right on the money. I used that exact spark tool and the spark is not blue. It will jump a good 1/2" but it is a bit orange. When I rev it up the spark remains the same. It sure feels like an ignition issue. I really appreciate the help from all. I will keep at it. Thanks!!!

So just to be clear, when you try to duplicate the low power, the spark stays strong?

The orange color doesn't bother me so much, as it's still jumping a 1/2" gap. I guess that could be a sign of a weak coil, but I'd think that'd show up during the low power with the spark tester.

maybe do a relative compression test just to make sure the base engine is decent. a 1994 is getting pretty old these days.

Ok , so , I replaced the COMPLETE distributor assembly (new), set timing . NO DIFFERENCE. I heard a tic while road testing. Put a bar on each valve cover to my ear. Could definitely hear a tap on the pass. side. Removed valve cover , found nothing wrong. I did a compression test when all this started . 150 across the board. Vacuum is about 16 . That should be a bit higher. It starts,idles perfect. As soon as the rpm,s come up about to where it shifts , you can hear the engine start to go flat. Get to a hill and it seems like it is on 3 cylinders. Has anyone ever seen the variable tuning gizmo on the top of the plenum cause any issues? This thing is really throwing me for a loop. Also checked TDC and timing mark . Doesnt appear chain jumped. Any other ideas???

As for back pressure, when this occurred driving down the interstate, my son cut the exhaust in front of the cat. It hasnt run right since it exploded the muffler that night on the interstate. We assumed the cat was plugged, but cat was empty. Since then replaced muffler etc. The fuel pressure was at low end of specs and flow seemed weak. Just replaced (with new) tank, pump,send unit and lines to filter . Fuel pressure/flow now good but no change to issue.

Saga continues but there may be light at the end of the tunnel. Unhooked O2 sensor and ran in open loop to rule out ECM. No difference. I went back to the vacuum gauge. I was checking to see if there was an exhaust restriction , making sure the cat parts didnt get stuck in the tailpipe. After slowly taking rpm,s up to 3000 gauge came back to 17 3 or 4 times, then needle started not coming back up . Removed tail pipe and the same think happened again. The first few tests were perfect and all of the sudden the tone of the engine changed, and vac. gauge was showing exhaust restriction. And when held at 3000rpm, engine started to have an occasional miss and gauge would quickly twitch slightly. So I think something is going on with the valve train. Valves sticking or out of adjustment due to wear. I assumed they were the same as the older chevy V8. Set them running till they get quiet and go 1/2 to 3/4 turn more. AllData says you torque down to 20 ft.lb. (non adjustable). Any suggestions.

Moved your post to repair section. If you want me to add this to your previous topic on this vehicle let me know.

Yes please. I am new to this forum stuff. Thanks !!!

No problem!

PROBLEM SOLVED. It was the Y pipe. Yes the Y pipe. I missed this early in the tblshting. I saw the slight flat spot just before the o2 sensor but didnt think much of it. It collapsed on top maybe 30%. It would only show up as an exhaust restriction on the Vac, gauge after it ran about 5 minutes. The second time I tested it for restriction i tested it more and found it. Also hooked up an exhaust pressure gauge to O2 sensor port. With this installed I could not duplicate restriction. It appears that with the sensor not in the pipe , it was less restriction. Since the flattened pipe was just after the two pipes came together and the O2 sensor in the pipe immediately after that maybe there was some reversion going on there also??? Replaced Y pipe and now it runs great again. Thanks to all for any advice given !!!!

Wow, I sure didn't see that coming! Congratulations on the fix.

So, I believe you about the Y-pipe being restricted, but I'm trying to connect that to the muffler blowing out. :blink: Unrelated issue?

Yes , that has crossed my mind. Not sure what happened there. The original O2 sensor was distorted/slightly melted . I assumed my son did that when he heated it to take it out . But he tells me he only heated it a little bit to get it out. If the O2 failed/ got too hot , maybe it dumped too much fuel ?????

kvnber4 wrote: Yes , that has crossed my mind. Not sure what happened there. The original O2 sensor was distorted/slightly melted . I assumed my son did that when he heated it to take it out . But he tells me he only heated it a little bit to get it out. If the O2 failed/ got too hot , maybe it dumped too much fuel ?????

Could be! Well, I'm sorry our advise was off base, but I'm still happy it's running right again.