I know it's not a 3rd gen, but I'm hoping that y'all can help me figure out what is going on with this thing before I finally lose my last marble.

It idles with a slight miss, when I give it gas to get it moving it has very little power and will actually backfire through the TBI if I try to give it more than about half throttle.

So far I have changed out the distributor(too much play in the distributor shaft), cap, rotor, plugs, wires, and coil while I was at it. I actually had to return the new distributor once because the first one still had about 50 thousandths end play.

I have changed the fuel pump and wiring harness from the engine to the fuel pump because I was having issues with it getting full voltage when I checked it..now I have a steady 10.5 lbs at idle, on the gas and even while it's backfiring.

I did a complete compression test(all cylinders were from 140-160), vacuum gauge readings are good(19 psi at idle, drops with throttle, comes back to about 25 at steady throttle, and goes back to 19 psi at idle again). It did have a stuck valve when I first cranked it over, but freed it up and replaced the pushrods on that cylinder for good measure.

I purchased a aldl cable to hook it up to the lap top and got all decent numbers, cts temp verified with thermocouple, good oscillation of the 02 sensor when warmed up, goes into closed loop fine(no power and backfiring happens in open and closed loop), the only number that goes screwy is the Spark Advance number(goes as high as 28098 degrees advanced). I haven't figured out a way to check to see what is going on while driving with timing to verify if it actually is advancing like a mad man yet. I've done quite a bit of reading and can't figure out what would cause this number to jump like that.

Trying to think what else might be helpful in the diagnosis. Fuel spray looks good, no drips. I've checked with known good MAP sensors, the new distributor came with new pickup and module(checked ohms on pickup), I've rechecked all the grounding points that I can find because of the rust on the truck, It's showing no codes at all, though I have disconnected sensors and got the corresponding engine code set for MAP, TPS, and CTS sensors so the diagnostic programs in the computer seems to be working. I have double checked the distributor timing mechanically(verified top dead center with valves closed matches up with tdc mark on damper with distributor at no 1). Plugs still look good after about 100 miles of limping the truck around(not sooty or wet). Changed thermostat when it took too long to get warm in the cold(used stock 195). I built a blockoff plate for the EGR to eliminate that as a vacuum leak/driveability problem. Can't think of what else.

thank you for any help that you can give me. I'm leaning toward a computer issue, but don't want to replace it for nothing. I have live scans of the engine running and cutting out in Tunerpro if anyone would like to see them.

 

Did you test the plug wires for continuity? I'm always getting new plug wires where one has a short in it. Make sure they are all securely on the plugs as well - had one pop off one time just enough to cause a miss and backfire - had to crimp the connector inside the boot a tad with needle nose pliers to keep it on. Only other thing I can think of is firing order, and making sure you hit TDC on the compression stroke as opposed to the exhaust stroke when you stabbed the new distributor.

 

I have checked my plugs and they all were the nice gray color, but I didn't actually check wires for continuity. I can do that and report back. I did the check for no 1 by watching the intake valve close, so I think that is good, but I can check that again as well. Thanks for the reply will report back.

 

Here are some datalogs from my truck that leave me scratching my head.


This screenshot was taken while powerbraking the truck and as you can see when I agressively hit the gas the spark advance graph goes vertical...have no idea why this would happen. Any help would be greatly appreciated.

 

i had that problem with my camaro it was the ignition control module

 

10.5 psi seems a little low for fuel pressure, I would try bumping to around 12 psi and see how it reacts.

 

This is the 3rd ignition module/distributor that has been in the truck, though 2 were from the same manufacturer(replacement distributor for the too much endplay the first one had), I guess it's possible that they've been bad. I'll try to get this one tested at the parts store to see what happens.

I don't have an adjustable fuel pressure regulator, but I have pinched the return to bump the pressure over 11 and the driveability didn't improve any.

Thanks for the replies.

 

Did you check to see how much vacuum you are getting? Vortec heads are famous for cracking on the exhaust valve down the runner and could cause vacuum issues. Also check crank position sensor.

 

Ignition module tested out good, pulled the plug wires one at a time to check to see if I had a bad cylinder, all caused about the same drop in rpm/vacuum.

I have about 18.5 in of vacuum at idle and it drops as it should when I give it throttle steadies out at constant throttle and tops out around 24 in as I let the throttle snap back to idle before coming back to the 18.5.