87-91 chevy TBI issues, "Code 42"

[Vbb199]

Joe did an excellent job of explaining the circuit. The only thing I’ll add is that a bitchy (not good but not totally failed) pickup coil can trip this code intermittently. It’ll test within spec, but it’s gone when you actually go to use it. From what I’ve seen online, this isn’t a super common problem, and it sounds like this is a very hard code so I’d bet everything the issue is in the actual wiring, and I’d do every voltage probe and continuity test before saying it’s the ECM unless you have a good spare lying around and want to play with it. My experience, though, is it’s not gonna let you win that easily.

I've got hoards of spare 87-91 parts to play with, but I did move that plug wire, checked connections visually, didn't see anything out of place....


Didn't take it for a test drive, but I will tomorrow.. I got too busy messing with my k5 after I got the exhaust installed on the burb lol.


I'll post results tomorrow, maybe I can cause it to trip a code, if so, I may swap out dist's and see if anything magical changes.
I got time, parts, and a tiny bit of money to play with!

 

[Vbb199]

Moving the #1 plug wire away from the module harness was not the fix, replacing the iginiton module with another one from a good working distributor was (for now), one thing that's definitely worth mentioning is, the truck fires right up, no excessive cranking (we're talking it used to fire up after 3 or 4 seconds of cranking)

Now it's just 1 second.

ALSO, rpms goes way higher and makes power the entire time, and when cruising 55, if I go to WOT, transmission kicks down and the motor pulls real hard unlike before.

That very well may have been the issue, but I'll continue driving it and see if I get another check engine light.

Iginiton module plug crumbled when I unplugged it, but I just replaced that with another good one.

@JoeR Jr

 

[JoeR Jr]

Excellent. Glad that worked out!
Joe

 

[Vbb199]

Luck would have it, now code 43 has set.

Which is knock sensors or their respective wires, or ESC module.

 

[1987 GMC Jimmy]

Does it set after a few minutes of driving or as soon as you start it up?

 

[Vbb199]

Does it set after a few minutes of driving or as soon as you start it up?

I know what you're thinking lol.

The ECM's self test of the knock sensor.
I get it after engine reaches temp and I've been driving for a good bit.

I have checked ESC, it is grounded. I have checked the knock sensor wire, I'm getting .004 ohms on it. So I don't have a reason to think there's a break in the wire.

I'm left with either a bad ESC module, or a faulty knock sensor.

Waiting on the misses to come out so I can show her what "retarded timing" looks like with the timing light while I tap on the block to see if it works as it should.

 

[1987 GMC Jimmy]

Haha, yep. I will say that I adjusted timing one time on mine, and it went crazy when it came time for the self-test. Jerking, bogging, codes thrown, and ultimately dying. I hadn’t disconnected the battery to let it reset after messing with the timing, but once I did that, it was fine. From then on I knew to reset the ECM after fooling with timing. Some of them are really sensitive to any little thing that arises, and some don’t care. Fickle creatures they are.

 

[Vbb199]

Haha, yep. I will say that I adjusted timing one time on mine, and it went crazy when it came time for the self-test. Jerking, bogging, codes thrown, and ultimately dying. I hadn’t disconnected the battery to let it reset after messing with the timing, but once I did that, it was fine. From then on I knew to reset the ECM after fooling with timing. Some of them are really sensitive to any little thing that arises, and some don’t care. Fickle creatures they are.

Dad set timing when he dropped this crate HO motor in, I trust it was fine. It carried us from NC to Maine, and everywhere in between time and time again with no issues.

I'm not gonna say "it's the gospel"

It's more so, tribal knowledge that the truck once upon a time ran like a champ with zero issues.

DAMNIT does parking something next to the woods for 6 years reek havoc on something.

 

[1987 GMC Jimmy]

Haha, try 15 in the bayou. “Make sure there aren’t snakes or raccoons living in there when you open the door.” :|

You must be registered for see images attach

But yeah, that stuff happens. I’m sure you’ll trace it down. Luckily there’s only a finite amount of things that it could be, and you already know what they are.

 

[Vbb199]

I did have to chase off a snake, sprayed that ****** with carb cleaner in the face

He dropped off the top of the trans and bolted.

 

[1987 GMC Jimmy]

I did have to chase off a snake, sprayed that ****** with carb cleaner in the face

He dropped off the top of the trans and bolted.

I can usually identify snakes, and I’ve never had a run in with a venomous one, just plenty of the non-venomous kind, and I’ll either let them go away or pick them up and toss them away. They don’t bother me so much. Wasps on the other hand...

 

[Vbb199]

I can usually identify snakes, and I’ve never had a run in with a venomous one, just plenty of the non-venomous kind, and I’ll either let them go away or pick them up and toss them away. They don’t bother me so much. Wasps on the other hand...

Yep, lots of wasp nests in the truck, rat piss and turds, nests in the AC vents. Good stuff.

 

[Vbb199]

Tried showing the misses the timing indicator, she couldn't see it (vision isn't so well) lol

So.. I guess I'll wait on my old man to return from church.

I will add that the indicator was almost out of sight under the water pump, and then after running for a little bit, it became visible and was close to the stamped timing bracket.

The dampener's indicator was appearing just after the the stamped bracket. As in advanced timing. Makes me wonder if I switched back to base timing again

 

[Vbb199]

Alrighty.

Dad finally arrived.
I had him check with the light while I tapped on the side of the block next to the knock sensor, timing does not retard.

So I must have a dead sensor.

 

[Vbb199]

Fixing to go to the parts store to get another. I'll install, reset ECM and then post back with results.

On the other hand, I checked TPS and MAP while I was waiting around..

Key on, engine off, I got .7v with throttle closed, slowly opening the throttle, at WOT I got 4.7v.

On the MAP, I believe I was getting 4.7v key off?

Turned truck on, I was getting .7v truck idling, wicked the throttle and it jumps to roughly the same as WOT tps voltage, like... 4.5-4.8?

Should be OK on TPS and MAP. Just wanted to verify those were OK.