For some reason I thought this vehicle is a 95 Silverado. But I don’t see “Silverado” in the list for 95 Chevy Trucks. Is it a C1500 2WD? 5.7L engine w/ TBI?
Your module can fail to increase the dwell as rpms increase. An old tach-dwell meter can test that as can many newer DVOMs using their % function. Of course an oscillascope is the most accurate test of the ignition system. Of course disconnecting the brown wire to set basic timing might set code 42 ,it’s been quite a while since I chased down codes on those.
Code 42 Electronic Spark Timing will be set whenever you check timing. The procedure for adjusting timing is to warm up the engine, disconnect the tan/brown wire connector and adjust timing to 0. Then shut off engine and reconnect wire. This defeats the timing control and allows base timing to be set. That code will remain in memory until the battery is disconnected or the code cleared using a scan tool.
@GeorgeSanJose
“95 Z71”
Most likely a 1/2 ton 4x4 Silverado with the 5.7 TBI small block V8
I have not replaced the crank position sensor yet. It is definitely a 95 Chevy with the Z71 package designated K1500. 4X4 5.7L w/TBI.
I was reading that the pickup coil and crank position is taken from the distributor shaft. Does that sound right? If the crank sensor needs replaced, wouldn’t that mean removing the distibutor shaft?
Sorry, I just read some other comments and I thank you for them. Asemaster has stated timing procedures. I did reset timing in the way you described because I had to time it when I did the top overhaul. When the problem of stumbling occured I checked the timing and it was advanced 4 deg. I just fixed it to a base of 0 deg TDC. I will clear the code and see what I get.
From what I can tell, you are right OP, there’s no crank position sensor on this engine. Instead the crank position is inferred from a sensor inside the distributor. Part of the ignition control module I presume. That’s why it is critical the base timing be set correctly, since if the base timing is set to a known position (0 deg I think on our vehicle) , then the computer can figure out the crank position given that it knows the distributor position.
If the problem isn’t the 4 degrees too-far advanced base timing you mentioned, It’s possible that sensor inside the distributor isn’t working, or is a little goofy at higher rpms.
That’s the way it works on my Corolla too, similar vintage of vehicle. No crank sensor. I have to set the base timing when I do a tune-up, and once that is accurate the computer figures out the crank position from the distributor shaft position. On my Corolla there are specs for the air gap distance between the moving and fixed parts of the sensor arrangement inside the distributor. Perhaps the gap inside the distributor has widened or is clogged with gunk on yours. Take a peak, see what the situation is.
You could have other problems that are causing this symptom, but I think the above comments are a good place to start.
If you want to take a chance the problem lies elsewhere, what I’d probably do in this situation is make sure the throttle body and MAF are clean (if that engine uses an MAF), and/or if it uses a MAP, vacuum test that device to make sure it is holding vacuum and hasn’t sprung a leak. I’d also check the accuracy of the throttle position and engine coolant temperature sensors…
Edit: If all that didn’t discover the problem, I’d hook up a fuel pressure gauge on the rail, arrange it so I could see the gauge while driving, and watch what it is doing when this symptom occurs. If it is dropping pressure suddenly right when the symptom happens, voila, you know what is wrong.
I went out and drove it today after retarding the timing to the proper 0 deg TDC. After a day of running I haven’t encountered the stumbling and there are no codes showing. This does not have a MAF but the MAP sensor is where I will go next if I have the issue again. I did not get it out on the highway yet. A tid bit I did not mention about the symptom is that when it shifts into OD it only stumbles when I am adjusting the throttle in for a small acceleration or to maintain speed on a hill.
GeorgeSanJose: you mentioned the coolant temp sensor. After I got the heads and all done I replaced the block coolant temp sensor. Since then my temp is registering around 160F instead of around 190F as before. I replaced the thermostat thinking they gave me a cooler opening stat but it is reading the same temp on the instrument cluster still. Any ideas? A mechanic told me after all the rework I did it could be running the engine cooler. I don’t agree.
The sensor in the block (head) is for the temp gauge only. The coolant temp sensor for the engine computer is in the intake manifold.
Thank you for the clarification. Any idea why my temp gauge would be reading so low?
A poor ground will cause all the D’arsonval analog gauges to read low. If the voltage is indicated below 13 volts while the temperature is indicated well below 190 and the fuel gauge never reaches the full mark the instrument cluster ground may be corroded or loose.
One thing that is certain, the engine has to be running at its designed temperature to function as the designers specified it to function. A while back my Corolla started running at a slightly less engine coolant temperature due to a faulty thermostat, and I got some difficult to diagnose problems, like high idle speed and jerky performance, especially noticeable at low speed. I didn’t realize it was caused the thermostat until quite a bit of diagnostic time later.
Suggest you take on as first priority this coolant temperature issue. Determine whether it is just the gauge, or the coolant actually is 30 degrees too cool. If the latter, I expect fixing that will improve this situation considerably.
BTW, you are right, even a total rebuild of the engine wouldn’t reduce the engine operating temperature. If all the passageways were made free and clear and you installed a super duper water pump and free-flow radiator, increased the diameter of all the coolant hoses, all that would happen is the thermostat would close a little more. The coolant temperature would remain the same. That’s determined by the thermostat, not the engine.
That might be why I get gauge swings. Could you tell me where the ground is for my cluster? I used a temp laser and it seems to be running at op temp.
I don’t recall where the ground is on your truck @jonfly673. Most likely it is under the carpet/padding behind the brake pedal. Years ago Mitchell Manuals had excellent wiring diagrams and in recent years they went to Mitchell on Demand with online access to information and even a DIY site that may have what you need. It might be worth the investment. But for now I would suggest that you pull the carpet back and look closely at the floor above the level of your feet especially near the parking brake. If you find the ground remove the bolt, clean the contact area on the floor and the ring at the wire end and reinstall it. Good luck.