SOLVED! 1987 350 TBI Timing advance issues

[gmbellew]

on the ICM harness -

black and red wire should ground through the ECM all the time. verify it is grounded good ground.

black and tan should have 5v when RPM>400. under 400RPM, or with the wire disconnected by brake booster, the timing is running only on the ICM timing. this will cause poor performance. you said you tested this wire, but make sure all connections are good. code 42 is set when it is open.

purple/white is the RPM signal back to the ECM. not for sure, but if this wire is bad, ECM may never see RPM>400 and will not send 5v to the black and tan and let computer control timing. check this one out also.

white is the timing control coming from the ECM when RPM>400. voltage will vary when system is working and ECM is controlling timing. voltage should be 0 if it is running off the module. I'd expect you have 0v.

I'd bet you have a connection or wire problem somewhere in that harness. or potentially the lowish probability the ECM is bad (or a bad connection at ECM).

 

[gmbellew]

I think your ESC is working correctly based on your description of testing the knock sensor. I'd sort out the code 42 and see if that goes away..

 

[troyfolley]

I tested the tan/blk but no voltage. Probably because idle is at 2000rpm. I will check the wire and connector again. What rpm should idle be to set base timing? How can I get the idle down. I have always been told not to puncture the seal and just idle screw on TBI.

 

[gmbellew]

tan and black wire should have 5v if RPM>400. so high idle is a separate problem than your code 42.

jumper AB on ALDL for about 30sec to force IAC closed. then unplug the harness connector to IAC so it will stay closed. then see what RPM is after you start it. if it is still 2000rpm, you probably have a large vacuum leak or the min air screw (t15 or t20 set screw on front of TBI on driver side. behind a cover if never messed with, exposed if previously messed with) needs to be adjusted. I think turning it counter clockwise closes the throttle blades and clockwise opens them up more. adjust the screw until idle is in the 450-475 RPM range. then turn off the truck and plug in the IAC and restart and see if idle is correct.

if adjusting the set screw doesn't bring idle down, you have a large vacuum leak somewhere.

for the code 42, I suspect you have a wiring or connection issue.

 

[Rusty Nail]

I think you are in a rabbit hole OP.

Wont increase rpm given gas you say?
I say bad / missing ground that didn't survive the engine swap.
Verify 1) driver side cylinder head to firewall and 2)engine block alternator to passenger fender 3) Negative battery terminal to core support.

Hope this helps!
Good luck!

 

[gmbellew]

good point about grounds. several sensors ground by the thermostat housing. hopefully that is in place also...

 

[Rusty Nail]

P.S.
I think that is called "the big three".

 

[Rusty Nail]

*in certain circles.

NOT to be confused with Metallica, Anthrax, and Slayer... AKA : "The big three".

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[festina_lente]

This thread has renewed my desire to "downgrade" my old TBI 'Burban to a carb and mechanical fuel pump

 

[Bextreme04]

This thread has renewed my desire to "downgrade" my old TBI 'Burban to a carb and mechanical fuel pump

A vacuum leak is a vacuum leak whether it is a TBI or Carb. There's a lot of people that have "downgraded" because they couldn't figure out the problem, only to find they have the same problem with the "new" carb. Making sure you have clean power and ground connections is essential, regardless of fuel delivery type. Not having a vacuum leak is also essential.

For the OP.. it still sounds like a vacuum leak. I would suggest spraying down the whole intake area and especially the base of the throttle body to see if you hear an RPM change. 2k rpm idle is a hell of a vacuum leak.

 

[Vetal4]

*in certain circles.

NOT to be confused with Metallica, Anthrax, and Slayer... AKA : "The big three".

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No Megadeth?

 

[troyfolley]

I think you are in a rabbit hole OP.

Wont increase rpm given gas you say?
I say bad / missing ground that didn't survive the engine swap.
Verify 1) driver side cylinder head to firewall and 2)engine block alternator to passenger fender 3) Negative battery terminal to core support.

Hope this helps!
Good luck!

Grounds are as follows.
Core support to battery
Battery to alternator bracket
Bell hosing to firewall

I will check again for vaccum leaks at TBI. Previously I checked just hoses

Thanks all. I will let you know what we find

 

[Black01Z]

There should be a ground coming out of the harness close to the thermostat housing

 

[troyfolley]

Yes that is connected. There are 2 grounds from the harness connected there actually.

 

[Rusty Nail]

Nope...rotlfmao..no megadeth. Soz.

You must be registered for see images attach

Maybe we're onto something here OP.

Lookin at you: "bell housing to firewall".
Dude, we're gonna fix it. I hereby forwarded the motion THAT GROUND is insufficient and lies at the root of your issues.

My suggestion is to remove that ground strap, check and verify its' construction as good then move that end of it to an empty bolt hole on the back of the cylinder head (on whichever side is connected to fhe firewall).
Yes , you'll probably have to find a different/new bolt.

To be very clear, sandwiching what is to be the main engine ground eyelet between the transmission case and the engine block will not yield a sufficient engine ground.

That's my gamble. If you had a STUD with a nut on the end - the story would end there - but if doesn't. Amirite?