TBI detonation (spark knock)

[bucket]

Help me Jim, help me!

My '88 TBI 350 Burb recently developed a detonation issue under acceleration. The timing is set at 8* advanced like it has been for years. We always run 87 octane in it, this last fill up we tried 89 but it didn't really seem to help the issue.

The other day I was playing around seeing exactly what conditions make it detonate. Heavy throttle from a standstill was fine, but it popped up as soon as it hit second gear and it continued until I let off the throttle. In one instance, I was on the throttle and it was detonating... so I backed off some (not all the way) and it was still detonating... then without letting off the gas anymore it just instantly went away. Kind of like the ESC just instantly starting working or something.

What should I be checking out? There are no codes stored.

 

[Jims86]

Help me Jim, help me!

My '88 TBI 350 Burb recently developed a detonation issue under acceleration. The timing is set at 8* advanced like it has been for years. We always run 87 octane in it, this last fill up we tried 89 but it didn't really seem to help the issue.

The other day I was playing around seeing exactly what conditions make it detonate. Heavy throttle from a standstill was fine, but it popped up as soon as it hit second gear and it continued until I let off the throttle. In one instance, I was on the throttle and it was detonating... so I backed off some (not all the way) and it was still detonating... then without letting off the gas anymore it just instantly went away. Kind of like the ESC just instantly starting working or something.

What should I be checking out? There are no codes stored.

Check the screws holding the module down in the dist.
Check the wiring and plug to the ESC Module.
Maybe its an issue with a batch of gas too.
TBI truck timing tables are restricted to a max of 29* total timing in most OEM tunes, with it advanced to 8* for base, you are giving it 37* in some cases, but unless the ESC is failing, I would venture to blame fuel, or it may even be time to use a good top end cleaner for carbon build up.
Also, just for ***** and giggles, check the red and black wire in cavity C on th esc plug with cavity B7 of the small ecm plug for continutity(make sure you unplug both plugs) and check to see if there is any continuity between that wire and chassis ground.

 

[bucket]

What top end cleaner do you recommend? My wife says the problem has only existed for the past two tanks of fuel. This last time was 89 grade and she says it seems to be quieter than it was with the previous tank. Not sure if it's just in her head though, I figured the problem would either exist or it wouldn't.

I had the air cleaner off to swap out the IAC valve (which I still have issues with) and all the connectors appeared good at that time. I'll check them again and test that red/black wire.

 

[Jims86]

What top end cleaner do you recommend? My wife says the problem has only existed for the past two tanks of fuel. This last time was 89 grade and she says it seems to be quieter than it was with the previous tank. Not sure if it's just in her head though, I figured the problem would either exist or it wouldn't.

I had the air cleaner off to swap out the IAC valve (which I still have issues with) and all the connectors appeared good at that time. I'll check them again and test that red/black wire.

I personally like Seafoam introduced through the eveap hose from the throttle body. Just foller the instructions on the can for top end.
GM partsmtop end cleaner is definately the **** too, if you can findna dealer that can manage to keep it on the shelf.
Also, if its less priminent with 89, might dissapear with 93(ouch$$$). kinda points to compression jumping up a bit.
Have you pulled the IAC plug and the ecm plug for those wires and checked those as well? Same check as the red n black wire. Its the big plug on the ecm Cavities: C 3, 4, 5 and 6.

 

[bucket]

I have Seafoam on the shelf at work, but I can have the local Chevy dealer send the GM stuff right to the shop too.

This IAC functions, just not correctly. It's very retarded and requires a little throttle on startup but then it idles at the correct RPM after that. I think I'm going to try a GM IAC valve and go from there. I've never remembered to check it while I'm in there, but I can just about guarantee there's a worn throttle shaft causing some of the trouble too.

 

[Jims86]

I have Seafoam on the shelf at work, but I can have the local Chevy dealer send the GM stuff right to the shop too.

This IAC functions, just not correctly. It's very retarded and requires a little throttle on startup but then it idles at the correct RPM after that. I think I'm going to try a GM IAC valve and go from there. I've never remembered to check it while I'm in there, but I can just about guarantee there's a worn throttle shaft causing some of the trouble too.

Gotchya...has the harness ever been changed? the reason I ask, is that I have dealt with some trucks where the owner unknowingly installed a 454 harness.
The two blue wires for the IAC are swapped on a BBTBI...causes all kinds of headaches when running a 350 chip!

 

[bucket]

The harness is stock for the truck

My wife got some 89 grade at a different filling station today, it didn't seem to help anymore than the 89 from the other station. I should have some time in the next few days to check things out.