Miss when trying to hold speed

You are my last hope. I have a 1994 GMC Sonoma 4cyl. SLS with 5 speed tranny. When I try to hold it at speed, it misses. For example, if I'm going say 15 to 20 mph in 2nd gear, it misses and jerks and cuts out, etc. Same if I'm in 3rd gear and trying to hold my speed at around 20 to 25/30 mph. It even does it in 5th gear at speeds under 50. I think it does it at say 60, but it is not nearly as noticeable. I have changed plugs and wires from NAPA. I have checked all vacuum wires and connections that I can get to. I changed one of the vacuum lines. No 'Check Engine' light. I have changed the thermostat and the O2 sensor. Nothing is helping. I keep suspecting vacuum leak. All plugs and wired have spark. I had a computer put on it and it told me about the thermostat and O2 sensor. That is the only problem the computer showed. There is no miss at idle. If I try to pick up speed slowly, it is there. If I step on the gas as in normal driving, the miss is not there. Most people that drive this thing never notice it unless they try to hold the speed like in town on the city streets.
Can anyone please give me some ideas?

Hi, welcome to Automotive forums. Is there by chance a vacuum advance on this vehicle? If so unplug it and drive it. If the trouble goes away then there's trouble with the rotor to cap relationship. If not then we still ask question and scratch head.

Thank You for your response. No, this vehicle does not have a vacuum advance. Any further ideas, Please?

If plugs and wires are good and good compression.
Check for good hot blue spark from coils when hot.
The coils are bad about losing good spark when hot.

Thanks. I will follow your advice, but it does it when it's cold also, if that makes any difference. I have wondered about the coils, but I don't know very much about this electrical stuff. I've run into brick wall after brick wall with this issue. If I could afford payments, I would get rid of this problem (although I do like this truck). I don't think I mentioned that I have 268K on this vehicle. Again, if that makes any difference.

Check for good spark hot and warm and cold.
Low fuel pressure can cause misfire to.
Good AC Delco plugs?
Good Fuel pressure?

I will check for spark tonight. But I'm lost on how to check fuel pressure.
I must leave, but will return in a couple of hours. Thanks for your help.

I will check for spark tonight. But I'm lost on how to check fuel pressure.
I must leave, but will return in a couple of hours. Thanks for your help.

To check for fuel pressure you will need a fuel pressure gauge and look for a scrader type valve on fuel rail to hook up gauge.

Specs are 9-13 lbs on fuel pressure.
If no test valve to hook gauge on you will need to adapt to fuel pressure line.

Unfortunately, I do not have the equipment necessary to check the fuel pressure. Is it possible that I could just have a vacuum leak? That is kinda what it acts like. But then again, I don't know much about this issue, so any additional advice will be more than welcome.

I did check for spark at all available spots, and it seems to be proper. So , again, I just don't know where to go next.

I did check for spark at all available spots, and it seems to be proper. So , again, I just don't know where to go next.

Good AC Delco plugs and cap and rotor.
Good compression.
Good hot blue spark to all plugs cold and after it is hot.
Any check engine lights?

Parts places rent or loan or sell fuel pressure gauges.

Can you rotate distributor body about 10 degrees (of crank travel) and test drive? Or retard tioming 10 degrees and drive it once. Rotate distributor (if it has one) the same way it rotates.

If it doesn't have distributor then we should look at fuel trouble or plugs/wires.

Thanks for the reply. No distributor. Plugs are AC and the plugs and wires are new from NAPA. Now you're beginning to see my problem. I have no way to check the fuel pressure. This thing is about to drive me nuts. Could it possibly be the fuel filter? It is about less than a year old. Spark is good everywhere. EGR maybe? I can move the diaphragm with a bit of effort with my fingers. If I do that with the engine running, it wants to die when I push the diaphragm in. There is no difference at idle if I disconnect the vacuum line at the EGR. I keep leaning towards vacuum because at about the same time this all started, I lost the use of my Cruise Control and the switch that chooses what I want to do with the heater. I could put the control on heat, vent, defrost, anything and the air would only come out the heater. I found a weather slit in a vac line and replaced the complete line. That fixed the problem except that now I don't get anywhere near the same force of air that I did before. If I use the vent, the air only comes out about 1/2 the force that it did before. And it takes forever to start blowing out the vents. If I have the air blower on full blast and turn the knob to vent, it never does come out the vent. I must have the blower on slow in order for it to switch over from anything else and blow out the vents. I have checked all the vac lines that I can get to, but that didn't help any. And, I my be waaaay off base about the vac lines. Please, help me some more if you can. My gas mileage has dropped from 28 in town to around 22 to 23.
Also, there 'check engine' light has never come on. The fella that put the computer on it said that if no lights come on, then it will probably not show any problems using the computer. He was correct. All it showed was faulty thermostat and bad O2 sensor. Both have been replaced.

You may very well have a vac leak , but I can't see that causing your symtoms while driving. It may be worth checking resistance on the new napa wires.

Exactly what would I be looking for with the wire issue? If there is a vac leak, would it be not as noticeable at high RPMs on a four banger? Thanks for the reply.....

Thanks fellas for trying to help me with this issue. It is still doing it despite what you have suggested. And I'm no longer getting any responses, so I guess I will close this thread. Again, thanks for your help. I tried.....

Thanks fellas for trying to help me with this issue. It is still doing it despite what you have suggested. And I'm no longer getting any responses, so I guess I will close this thread. Again, thanks for your help. I tried.....

You need to check that fuel pressure.

You cannot leave home with good fuel pressure.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

Exactly what would I be looking for with the wire issue? If there is a vac leak, would it be not as noticeable at high RPMs on a four banger? Thanks for the reply.....

Look for 1 wire with a much higher reading then the others. I can't say what's normal because it has been years since checking wires. It may also help you find some opens or gaps.

Thanks again. I'm sorry it took so long for my response. Had to spend some time in the hospital. Anyway, there was no noticeable difference in the reading of the wires. The only thing left is to take it to a dealer. And I'm not sure if the truck's value is high enough to pay for that. Thanks so much for your suggestions. If there are any more ideas, I would be happy to hear about them. I just wish I could check the fuel pressure. But I won't be able to do that one.

Hello to all that responded to my query. I apologize that I haven't been on for a bit, but I had nothing new to report. Now, I have. The problem has been identified . It is the "Clutch". Who woulda thought? Not me, for sure.
I again thank you for your interest and you help with this issue.

Hello to all that responded to my query. I apologize that I haven't been on for a bit, but I had nothing new to report. Now, I have. The problem has been identified . It is the "Clutch". Who woulda thought? Not me, for sure.
I again thank you for your interest and you help with this issue.

You are welcome and glad you found the problem.
But a clutch will not cause a miss.
What is the clutch doing or not doing?
Good Luck