Code 42

[SoCalSlaughter]

Stuck! Ive had a code 42 on my 1990 Suburban 5.7 4X4 for about 2 months now. Cold start the truck will run rough, once warmed up it will run good. If im at a stop light for a few minutes the check engine light will come on and the rpm will drop, but the truck will take off fine. Ive tested the ignition module, checked with another unit. Swapped ecu, same result, swapped whole distributor and same result. Pulled whole wiring harness out, checked all wires and grounds. Only thing I havent changed is the MSD ignition coil. Its about 3 years old. Oh and havent change the PROM in the ecu. Check the set timing connector and is all good. I cannot for the life of me figure this one out. I have an idea its the PROM because the last owner said it had a short in the ECM-B circuit for about a year and would blow the fuse and shut the truck off. So im thinking maybe the PROM is bad from being blown so many times within that year. That circuit is all fixed but it has the same PROM. Any ideas? tests I can do?

[VetteVet]

Check this thread.
..........http://67-72chevytrucks.com/vboard/s...d.php?t=411794

[VetteVet]

If that doesn't help here are some more.

..........http://www.google.com/custom?domains...D%3A1%3B&hl=en

[SoCalSlaughter]

Ive read threw all of those and gone threw my truck so many times, I have multiple wiring diagrams and ECU pin chart. Other thoughts Ive had is maybe a bad electronic spark controller, failing ignition coil, or bad PROM in ecu. I work at a autoparts store so Im able to bring the Wells tester home with me and test sensors and modules (except spark controller) Im stumped

[VetteVet]

If you swapped the ECU did you not swap the prom as well?

[SoCalSlaughter]

I swapped the PROM from my old ecu into the new one with same result. I was told to try changing out the ignition coil, Im going to try that and cross my fingers.

[68c10airstream]

Fixed many code 42 as a gmc dealer mechanic. Here we go, i went to gm schooling, set 1' set 2. That's specialized electronic training . From 20 years ago my memory is this; code 42 is est bypass circuit fault. When customer's truck would come in it was from either a result of a tune up or motor swap. I live in the salt belt and this is what happens. When you set the timing during a tune up or motor swap the dist. will sit in a new spot. the new spot was NOT CLEAN. Long story short is this, start truck up, scratch a spot on the neck of the distributor until it is shiny and bare. Take a good volt meter on dc volts scale, touch the negative lead of the meter to the negative battery post (shine this up too so you get a good reading) and touch the other lead (positive lead) to the shiny spot on the distributor neck. IF IT READS ANY VALUE YOU WILL NEED TO THIS, remove hold down and clean the underside, clean the base of the distributor where the hold down touches, clean the block where the hold down touches, clean the bolt and retest. If it reads anything do it over again. I probably fixed 20 of these in my days and never put a part in. Here is the theory. When you go into full spark advance during light throttle the module is working a little harder than at say idle so the light usually comes on off idle. Your ignition module mounts to the distributor body for the ground to complete it's circuit. Now the distributor is needing a good ground and has to get it through the hold down, if it can. When it can't get a good ground, check engine light comes on. I used a fluke model 87 which is a great (and expensive) meter and never failed me. Good luck, Brian F.

[68c10airstream]

I should have added this, the volt meter is trying to supply the needed ground as it is positioned parallel to the vehicle ground circuit. The meter reads a value because it is supplying the needed ground. This is called a voltage drop test.

[SoCalSlaughter]

Wow, thank you for that. I do have a new distributor in the truck, bit Ive never cleaned the hold down clamp. The manifold is a new Edelbrock performer 3704. I had an idea from looking at the wiring diagram, seeing the ignition control module need a ground threw the distributor. Do you think this would also cause a miss at idle?

[SoCalSlaughter]

Did the test, negative lead on negative battery post with positive lead on distributor base showed 1.2 ML, negative lead on battery negative with positive lead on ground on motor showed .8 ML, the hold down clamp does look a bit dirty and greasy

[SoCalSlaughter]

Sorry, engine running! haha the 1.2 was with engine off, key on. Engine running was about 14v. I took the hold down off and its pretty nasty. cant see any bear metal, ive been cleaning it up with sand paper, Im hoping this is the cause of all my problem.

[SoCalSlaughter]

I have a set of Accel 8mm wires on the truck, they are about 2 months old. I changed the plugs at the same time. Removed a few plugs a few days ago and they all have an even burn. A slight orange color, not one plug sticks out more then another.

[68c10airstream]

lets get the check engine light issue resolved first with the timing set properly. Then if issues persist we can go from there. Brian F.

[SoCalSlaughter]

At the moment the timing is advanced to 8 degrees, I cleaned the hold down clamp but I havent had time to test again and see if its properly grounded. But the 14 volts I was seeing on the first test, how bad is that reading?

[68c10airstream]

if my memory is correct i believe with it idling in park warmed up, disconnect the single wire connector at the firewall , engine speed then drops down and labors then set to 0 degrees, 8 degrees seems too much, but it's memory recall at this point. If you did the test meter correctly it should read basically nothing, if the ground is proper, because the meter and leads won't act as a ground supply. Disconnect the battery to clear the memory, set timing to spec and drive it. If the check engine comes on, check codes to verify and go from there. You have done a lot of work in the past on this so lets start with a clean slate. Good luck, Brian F.

[SoCalSlaughter]

I drove the truck to and from work today, no light!!! I want to do some more driving, longer period of time to confirm code 42 is gone. Ive tried setting the timing in the past and it seems to run rought when set to 0 and has no power.

[SoCalSlaughter]

What are the correct steps in setting the timing?

[68c10airstream]

memory check again, vehicle warmed up and running in park, disconnect the single green? wire connector at the firewall (behind the big black plastic cover) and then set timing to spec, i think 0 degrees. Double check the label under the hood as it states timing spec. When done shut of and then reconnect the single wire connector. This connector behind the big black cover runs horizontal and it appears along side a big harness (probably 1" in diameter). The black plastic cover is held down with 3 plastic nuts that stand out about 1 1/2" tall. Good luck Brian F.

[SoCalSlaughter]

The set timing connector on mine is the tan with black line. I unplugged it, started truck (while warm) set to 2 degrees btc, turned off truck, connected wire, pulled ecm fuse for 5 min, replaced fues and started truck up. Took it for a spin and seems low on power now. Doesnt seem to like being close to 0

[68c10airstream]

What was the timing spec on the label under the hood? If it is 0 and you have to set it at 8 btc and the harmonic balancer notch is legit, possible timing chain jumped 1 tooth. This will be a little more involved to check, but i will wait for your response before we continue. Does the truck still run doggy at 8 degrees and doesn't ping itself to death? How many miles? How long have you had it? I'm looking for a time line here, so if you say something like "no matter how far it is advanced it is doggy, and is has run better before" then we will be digging deeper. Not trying to scare you, but you have changed many parts, and a lot of work, so this can add to the madness of car repairs. I will want to know the status on the check engine light, is it on? what is the trouble code" etc. Let me know when you can. I'm working day shift (12 hr shifts) thursday through saturday so about the time i get to the computer it will be about 6:00 pm your time. Good luck, Brian F.

[SoCalSlaughter]

The check engine light is gone, been driving the truck the past 2 days. Truck seems to like the timing set more advanced. Its set to 2 btc and seems to run well. I bought the truck in 2007 with 112k miles on it. Its about to hit 159k. Did a double roller timing chain about 1 month ago, everything lined up well. The balancer is fine, took it off and lined it up to another balancer on a friends truck to make sure it hasnt moved.
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[68c10airstream]

That's great on 2 accounts, your truck is running good with no check engine light on, and my memory was pretty good. I finally got into alldata and found the spec and it is 0 degrees, at top dead center, so if you are getting by with 2 btdc, great. Now doesn't the crappy ground at the distributor make sense! I took that gm schooling in 1990, so it was a while ago. I have gotten a lot of mileage out of that repair, and learned a lot about the most neglected and forgotten circuit, the grounds. Out of sight out of mind. Your persistence paid off, and now you can pass the voltage test along to somebody else and be the teacher. It's great if you are right and it works, it sucks if your wrong. Good luck, Brian Fuller

[SoCalSlaughter]

Thanks alot!! I work at a parts store and ive already been passing on this new knowledge. Haha thanks again!!!
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