1990 chevy code 42 / Running rough
 

I have a 1990 chevy pickup with a 305 that originally had a v6 in it, manual trans, 2wd. Its reading a code 42 that according to what I read is Electronic Spark Timing (EST) Bypass Circuit fault. I really don't know where to start with this. Is there a simple fix to this? I'm new to these new fangled computerized vehicles. Any help is appreciated.

Re: 1990 chevy code 42
 

Have you checked or adjusted the timing recently?

A code 42 will get set when the set timing connector is unplugged to set the timing.

Does the service engine light come on while driving?

Re: 1990 chevy code 42
 

I haven't done anything with the timing, I just bought it recently so it may have been the previous owner. It comes on when you start it. I guess ill check for loose connections first.

Re: 1990 chevy code 42
 

Clear the code by pulling the ECM fuses for 30 seconds and then reinstall the fuses. Make sure the key is OFF when you do this.

Check the code to make sure it is gone.

Then start the vehicle and see if the code 42 comes back.


A code 42 is set when the ECM loses communication with the distributor.

Code 42 can be set by a bad distributor module, a bad ECM or wiring problems between the ECM and distributor.

A bad distributor module is a common cause for this code.

Many stores like NAPA in my area will test the module for free if you bring it in.

Make sure when you install or reinstall the distributor module that you use heat sink compound/grease under it. The grease helps dissipate heat from the module. Without it, the module will fail.

Look very carefully at the terminals on the connectors that plug onto the distributor module. Make sure they are not corroded and the fit tight (wire terminals not stretched open).

Re: 1990 chevy code 42
 

I pulled the fuse and the code still came up. I opened the glove box and half the screws were missing from the glove box and the ecm unit was plugged in and "floating" not really fastened down to anything.
I took the distributor module and had auto zone test it and they said it was fine and put some of the grease on it.
When I went to plug the harness from the distributor in to it the wires fell out of plastic connector so i put shrink tube on them separately and hooked it all back up. I'm still getting a code so ill have to flash it and see if its the same code.

The truck is running like crap, could that be caused by the ecm? On slow acceleration its running really rough, but on heavy acceleration is seems fine. I changed the gas tank and fuel pump recently could it be a fuel issue?

Re: 1990 chevy code 42
 

The free tests that are done to modules at auto parts stores don’t always find all bad modules. Some pass the test but are problem modules. Heat is a factor, and people experienced with testing modules will often test them a couple times to see if it fails as it heats up.

Check to make sure the set timing connector is not unplugged.

You could test the wires between the module and the distributor with a meter and an ECM connector pin-out chart.

The ECM could be the problem, but it is more common for a bad distributor module to set this code.

Fuel issues should not cause a code 42 but they can cause a code 44 or code 45. Be care full when you count the flashes.

Often fuel pumps get replaced on these trucks when the fuel pressure regulator spring breaks. When the spring breaks the fuel gets dumped back into the tank and the pressure will be low.

Re: 1990 chevy code 42
 

I"m not sure where the timing connector is. There is a single purple wire with a round plug coming out of the harness behind the distributor but I don't see a place to plug it in. There is also a single white wire with a connector coming from the coil area.
I'm thinking I may have multiple issues causing the truck top run poorly. I'm just trying not to just throw money at it and hope it fixes it.
Thanks.

Re: 1990 chevy code 42
 

From a reply I made in the past:

For 1988-1993 C/K pickup trucks
To set the timing you must unplug the set timing connector. The set timing wire, which is a single Tan wire with a black stripe/tracer on it, is located, under the hood, on the main harness that goes from the engine to the firewall, on the passenger side, sometimes underneath the black plastic cover (if it’s still there) that covers the junction block, relays & fuses. It’s near the top usually on the back side of the harness. From the factory there is what looks like black tape around the harness holding the set timing wire to the back of the harness. Doing this will set a code 42 in the computer. Clear the code after you are done checking or setting the timing.

Re: 1990 chevy code 42
 

Ok, I finally found the set timing connector. It was unhooked so I plugged it in and the truck still runs like crap. I'm going to go check for codes now.

Re: 1990 chevy code 42
 

I got the code cleared. I had to find a video on you tube to find the connector so I now I feel dumb... So now I found the u joint in the rear shot one of the caps off so its on to fix that.
Thanks for your help.

Re: 1990 chevy code 42
 

just for info,my 94 timing connector is under the dash,right where the wire harness comes through the firewall. its a 94 4.3 2wd 5 speed.

Re: 1990 chevy code 42
 

Thanks for letting us know what you found.


It sounds like your truck is keeping you busy.


Yes:
In 1994 the set timing connector was moved to inside the cab on the C/K trucks. This is another example of why it is important that people give vehicle information when posting. Many people are posting questions on the board here that do not have enough vehicle information so that a good answer can be given.

Re: 1990 chevy code 42
 

I replaced the distributor module and It ran better for a few days now its back to jerking and popping. Its still storing a code 13 and 42. The engine that was swapped in has no spot for an o2 sensor on the manifold... Im not concerned with mileage its a spare vehicle, I just want it to run better.
So having swapped out the distributor module what next to check for a code 42? Im assuming that means the ecm is bad.

Re: 1990 chevy code 42
 

next to the throttle body, on the passenger side,on the metal bracket,there is a small black box about 2 x 3 inches..with a plug on it,That is the EST module..change that with another..My roadmaster did the same thing,that fixed it.

Re: 1990 chevy code 42
 

I unhooked it and it didn't see to make a difference in how it runs. Im not concerned with engine codes so much as I want it to run better. I just cant tell if I have multiple issues or if its all tied in the code 42. Im also trying not to just shotgun it, im trying to be as cheap as possible lol. Thanks.

Re: 1990 chevy code 42
 

Did you put heat sink compound/grease under the module when you installed it? Without it module failure is likely, very soon.

Are you saying there is no O2 sensor on the truck?

Re: 1990 chevy code 42
 

I used all the grease that the new module came with.
Its supposed to have an o2 sensor but the truck has an older engine swapped in with no spot for an o2 sensor on the manifold.

Re: 1990 chevy code 42
 

If it keeps setting a code 42 you need to find the cause.

If it keeps setting a code 42 you need to find the cause.

Re: 1990 chevy code 42
 

I looked up the number on the ecm 16144288 and it comes up as a v6 but the truck now has a v8 is that going to cause any issues or do I need a ecm from a v8?

Re: 1990 chevy code 42
 

Yes that could be a problem.

It does look like the 16144288 ECM was only used with the V6.

My guess is if someone used that ECM then it most likely has a PROM for a V6 also.

Re: 1990 chevy code 42
 

mabey i can help with some pictures from my 1987 short bed with the TBI Hope this work this is from a 1987 truck first Year they really used the TBI and the 700R4

Re: 1990 chevy code 42
 

I cleared the code by unhooking the ecm. I bought an ecm for a v8 off ebay and installed it today and now the truck is running great! Hopefully it will stay that way for a while.