Miss when trying to hold speed

[poppyal1]

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?

[534BC]

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.

[poppyal1]

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

[MT-2500]

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.

[poppyal1]

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.

[MT-2500]

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

[poppyal1]

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.

[MT-2500]

Originally Posted by poppyal1

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.

[poppyal1]

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.

[poppyal1]

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.

[MT-2500]

Originally Posted by poppyal1

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.

[534BC]

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.

[poppyal1]

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.

[534BC]

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.

[poppyal1]

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.....