92 Suburban K2500 Heavy duty 4x4 454 no spark on start up

[Jeff C.]

Hello, Sure hope it's computer because I ordered one. Throttle body has new injectors, not rebuilt.
During start have fuel, spark at coil, key on hot to coil and out to module . Never any spark , just did timing chain, and O2 sensor in case, compression is plenty good enough for old beast 160, 170, 180 as you go back on both sides, yes crank long enough and have oil pressure. Power and grounds at ECM good , maybe one pink not but not sure it should be hot...
Going out now to try start without tan/black bypass connected to check for spark at top of cap
I'll be back ....Thank you
So, very slight spark off the top of distributor , without the bypass plugged in. Strong spark off the coil. Tach ever so slight reacts to go up but doesn't, with or without bypass in..
I got a BACKFIRE through the carb... then next try through the tailpipe. Second time though the carb, last time was at least a week ago and very gross dirty scum came up to the butterflys
If I have good enough compression then the timing should be correct with the timing chain /gear replacement right?
I'm going to recheck top dead center on damper, cyl 1 piston and distributor.
I cleaned the distributor connectors with elec clean spray even though they looked fine.

 

[cheavyk10]

Did you check the module? They are notorious for failing.

 

[Jeff C.]

Did you check the module? They are notorious for failing.

Well, its a new replacement distributor from AutoZone (they are very good to me) because the I thought new I just got might be )failing in between the module and cap (forget what that is called.) like the old one. That new one got the modules swapped a few times and I have a the one from that first new one distri and they checked it and it passed.....

 

[Jeff C.]

Did you check the module? They are notorious for failing.

Thanks for your reply. Please check my update edit to my original post and my reply to you incoorectly sent as a new post.
Thanks again!!

 

[Blue85]

spark at coil, ... Never any spark ,

Do you mean power at the coil, or that there's a signal going high/low? Or is your edited post saying that spark looks good now?

just did timing chain....I got a BACKFIRE through the carb... then next try through the tailpipe. Second time though the carb, last time.

Do you have pictures of the timing marks lining up? Just thinking this through, it seems like you could get the cam off by 180 degrees and build compression on what the ECU thinks is the exhaust stroke. The compression test would be good, but the spark would happen as the exhaust valve is closing, so no fuel or compression present.

If the distributor was out during this work, it's also possible to have that 180 or out, or really anywhere. Remember that 180 out will give the correct reading with a timing light.

 

[imiceman44]

Hello, Sure hope it's computer because I

Do you mean power at the coil, or that there's a signal going high/low? Or is your edited post saying that spark looks good now?

Do you have pictures of the timing marks lining up? Just thinking this through, it seems like you could get the cam off by 180 degrees and build compression on what the ECU thinks is the exhaust stroke. The compression test would be good, but the spark would happen as the exhaust valve is closing, so no fuel or compression present.

If the distributor was out during this work, it's also possible to have that 180 or out, or really anywhere. Remember that 180 out will give the correct reading with a timing light.

That happened to me and yes the symptoms do match

 

[Jeff C.]

Do you mean power at the coil, or that there's a signal going high/low? Or is your edited post saying that spark looks good now?

Do you have pictures of the timing marks lining up? Just thinking this through, it seems like you could get the cam off by 180 degrees and build compression on what the ECU thinks is the exhaust stroke. The compression test would be good, but the spark would happen as the exhaust valve is closing, so no fuel or compression present.

If the distributor was out during this work, it's also possible to have that 180 or out, or really anywhere. Remember that 180 out will give the correct reading with a timing light.

I think I have a picture of them lining up, the cam was never rotated and believe the crank was at cylinder 1 tdc, I checked the that the dist pointed correctly at cyl1 at the cap i ensured that it was at tdc with a endoscope watching the intake valve open during the downstroke and closed for the compression up stroke.
the strong spark during start is at the coil, the weak spark is at top of cap at cyl1.
i don't really understand the high low but both are hot to module on run. would the weak spark at top of cap be a 'low' but not a 'high'??? What about the backfires???
I should be able to pick up the 'new' computer today or tomorrow.
Thanks for your consideration

 

[Jeff C.]

That happened to me and yes the symptoms do match

Thanks, I hope you mean about the computer and that it was the fix because i pick it up today or tomorrow

 

[Blue85]

the strong spark during start is at the coil, the weak spark is at top of cap at cyl1.
i don't really understand the high low but both are hot to module on run. would the weak spark at top of cap be a 'low' but not a 'high'??? What about the backfires???

If the spark isn't making it through the distributor, you may have a bad rotor or carbon button/spring (should be under the center terminal). One pin of the coil primary is +12V. With no spark called for, the other terminal just floats to 12V. To charge the coil (before a spark), the 2nd terminal is pulled low for the dwell period (a few to several milliseconds), so it's kind of a square wave there.

If the distributor is 180 out, the rotor will point in the same direction as when it's correct. Remember the engine goes around twice for every rotation of the cam and distributor. Instead of sparking BTDC on compression, it sparks BTDC on the exhaust stroke. It depends on the spark timing and cam profile, but the exhaust valve is closing and intake valve opening around that time, so it can puff a little in either direction.

If you never moved the dizzy or cam while the chain was off, the spark should still be timed to the valvetrain, but you did have the opportunity to get the chain on wrong. Now, some random installed position would likely lead to poor compression, but if you did some interesting variation, like both marks up or both marks down, it could be like I described above.

 

[Blue85]

It sounds like the no start was your original problem and all the rest of the work was chasing that down. Sure, a bad ECU could cause it, but when the coil and injectors are being activated, a lot of parts are working right. It could also be a sensor problem, like TPS or MAP. Have you measured the fuel pressure? I feel like we need an emoji holding a sign reading "check the fuel pressure", it gets suggested so often.

 

[Jeff C.]

It sounds like the no start was your original problem and all the rest of the work was chasing that down. Sure, a bad ECU could cause it, but when the coil and injectors are being activated, a lot of parts are working right. It could also be a sensor problem, like TPS or MAP. Have you measured the fuel pressure? I feel like we need an emoji holding a sign reading "check the fuel pressure", it gets suggested so often.

Thanks for your time and consideration. Now I'm a little confused on tdc....I have it at the top of upstroke after the downstroke where the intake opens to pull in mixture.....
The back fires happened when the distributor was pointing 180 out/off/incorrect.
I think I have it correct.. I'm getting a weak spark off the dist cap on cyl1 when I remove the spark plug wire....
I hadn't thought of using the timing light on the spark plug wire during start.......I'll try try that.
It's a new distributor , I am getting spark through it but weak.....
Now how would the MAP sensor affect .
I believe I've got good fuel pressure because I get very good fuel spray when the injectors are connected. I do the testing to get a spark with them disconnected mostly because this all started when the spark must have stopped and the engine really flooded and it took a couple of days for it to dry out. I removed the plugs and cleaned them, really wet....
I bought new plugs but don't want to use them till it's running somewhat. I do have a new plug on cyl 1 (don't ask me why)
Thanks again
I'll be back.....after seeing if I try the timing light. I think without tan/black ECU bypass plugged in....
Also when i got the new injectors it came with a new pressure valve and gaskets and i put a new fuel filter as recommended....

 

[Blue85]

If the spark is weak, that gives you something to chase. I see 2 different styles of rotors for that distributor. They must work with different caps, and mixing them up could add an extra spark gap to the mix. Another issue that can cause a gap is if the timing is way off: the rotor isn't really pointed right at a terminal when spark is happening. That can also cause backfiring and such.

 

[Jeff C.]

Thanks for your time and consideration. Now I'm a little confused on tdc....I have it at the top of upstroke after the downstroke where the intake opens to pull in mixture.....
The back fires happened when the distributor was pointing 180 out/off/incorrect.
I think I have it correct.. I'm getting a weak spark off the dist cap on cyl1 when I remove the spark plug wire....
I hadn't thought of using the timing light on the spark plug wire during start.......I'll try try that.
It's a new distributor , I am getting spark through it but weak.....
Now how would the MAP sensor affect .
I believe I've got good fuel pressure because I get very good fuel sprat when the injectors are connected. I do the testing to get a spark with them disconnected mostly because this all started when the spark must have stopped and the engine really flooded and it took a couple of days for it to dry out. I removed the plugs and cleaned them, really wet....
I bought new plugs but don't want to use them till it's running somewhat. I do have a new plug on cyl 1 (don't ask me why)
Thanks again
I'll be back.....after seeing if I try the timing light. I think without tan/black ECU bypass plugged in....

If the spark is weak, that gives you something to chase. I see 2 different styles of rotors for that distributor. They must work with different caps, and mixing them up could add an extra spark gap to the mix. Another issue that can cause a gap is if the timing is way off: the rotor isn't really pointed right at a terminal when spark is happening. That can also cause backfiring and such.

View attachment 517381

Thanks for your consideration. It is a new complete distributor from AutoZone (replacement for a complete new one that was put in to fix a jumpy sparky tach from a failing of the section between the module and the rotor. They kindly replaced it in case it was failing. Didn't do the trick) On wednesday I did the top dead center on cylinder 1 and reset the pointing of the rotor (as it was off 180 and hence maybe the backfires) correctly (hell I hope so) and no more back fires. Good spark at top of coil, weak spark at top of cap cyl1 and none registering on the timing light.......
I picked up the new computer and it clearly says it is 'unlikely the computer that failed but another part of the system'. I've checked power and ground to the computer and cleaned the grounds by the thermostat. Always recharge the battery before attempting a start. The EGR valve isn't sticking but could be the control (but I don't think that would negate a start) . The MAP , same , would that negate a start?, don't know.....What other sensor would negate a start???
The tach moves ever so slightly on start but doesn't go up to show any rpm.
I'll be going out soon to recharge the battery , it would not take a full charge yesterday so maybe it's tired of all this. Mo'money??
I sure appreciate your feedback. I hope this new/rebuilt computer is the fix . More will be revealed.....right??