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Discussion Starter · #1 · (Edited)
My homebrew TBI conversion on my cj is acting up. It is backfiring and missing badly during acceleration and at anything over 2k rpm's:cussing: My first thought is the Electronic Spark Control module is toast, and I am just not getting the timing advance it is supposed to provide. My check engine light isn't on, but it may just be burned out.

I know the computer really wants a distributor reference pulse to trigger the fuel pump, but I bypassed that part of the harness so I have fuel pressure as long as the key is on. Would it be possible to throw in an old vac advance dist, or would the lack of a reference pulse just piss off the computer?

I am going to read up on it tonight and see if I can't get a better idea of what's going on. I appreciate any other ideas thrown my way:eek:
 

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Discussion Starter · #3 ·
No CPS on this setup. The TBI is basically built to be an 87 GM system.

Off the top of my head it's supposed to have these sensors:
O2
Knock
VSS
EGR valve
Temp
Oil Pressure

I have all the sensors except the VSS, and my EGR valve is not hooked into my motor due to my ancient intake manifold. Other than that, my system is pretty much complete GM TBI.

The ESC (electronic spark controller) controls the advance of the distributor. It will advance the timing right up until detonation is detected by the knock sensor. It then backs off the timing to provide the proper advance curve for the entire rpm range. It's a pretty slick system. In fact I have seen a moron I used to work with throw the distributor in, bolt it down and drive without even checking the timing. The blazer ran great, however when they were shown the proper way to time it by disconnecting the signal to the ESC they found it was so far off the base timing that it wouldn't even run.

Long story short. If it had a CPS I would suspect it. I think tommorrow I am going to go buy a new timing light and set the base timing again. Then find my DMM, and find out if my check engine light is burned out. Hopefully I can pull some codes and approach this thing systematically, rather than willy-nilly.
 

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So you are saying the system has been running good and now its developed this problem??

Do you have a way to check out the trottle position sensor??
Is it back firing or popping thru the intake?? Might be a lean condition miss fire.
 

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Discussion Starter · #5 ·
So you are saying the system has been running good and now its developed this problem??

Do you have a way to check out the trottle position sensor??
Is it back firing or popping thru the intake?? Might be a lean condition miss fire.
The system has ran great for the last few years. First on an AMC 304, then on a chevy 327. This just started as I was leaving Dan's Ferry for lost trail two weeks ago.

I will test my TPS as soon as I find my DMM:lame: . It should just go from .5v closed to 4.5v wide open. I will also try to run it in diag mode after I fix the check engine light. That will also tell me if its running lean.

It seems to be popping thu the intake, but there may be an occasional backfire. Really I need to get off the dang 'puter, and go start working on the thing:shame:
 

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It sounds to me like you have one of two problems, loss of fuel pressure or spark like you mentioned.

Check your fuel pressure, it should be in the 9-11 psi range, and check it while running and during the misfire condition. Inspect your plugs, cap, and rotor. While you have the cap off visually inspect the ignition module and pickup coil, and check the distributor shaft for wear. The early distributors had lots of problems with wearing out the upper bushing, then the shaft would wobble and start grinding away the teeth of the pickup coil. The shaft movement causes all kinds of erratic spark behavior. :thumb:



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Discussion Starter · #7 ·
Well I went to schucks to pick up a timing light, even though I knew I would pay way more than it was worth. While there the old parts guy asked what I was working on. I described the symptoms and he said, "sounds like you have an intake valve sticking".

Got home and set the timing, then verified the ESC was working. Started pulling spark plug wires and firing it up. Sure enough on cylinder #2 as soon as I unplugged the wire the backfire through the intake disappeared:mad2:

Pulled the valve cover and watched it idle. The rocker for the intake valve was barely wiggling at all. I've either got a shot cam, or a worn pushrod.

It's good to know what's wrong, but I've got so much on my plate right now I know I won't be able to swap out the cam anytime soon:mad:

Plus I gotta figure out what type of cam to put in the engine, and of course this brings up the question as to whether or not I just need to rebuild the whole :cussing: thing.
 
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