Erratic Idle when warm

[Marcos Beltran]

I have a 95 stepside V6, 4.3 liter TBI that im having some weird issue going on. About a year ago i had a problem when pressing the gas the the trucks rpm would raise up but the truck would not accelerate. Replaced TPS and fixed rhe problem. Well the same thing happened about 2 weeks ago and replaced the TPS cause it was under warranty and worked for a week and happened again. This time though it seemed to bog down and the idle would fluctuate. Cleaned the IAC cause it was dirty and it drove really smooth for about 25 to 30 minutes and problem came back. Replaced map sensor with a known working one and worked again for 25 to 30 minutes and problem came back. It seems like it has a vacuum leak but there isnt any and i would think that the vacuum leak would be there once you turn on the truck not 30 minutes later. So i started to suspect that it seems to start having problems once the truck is fully warmed up. So i suspected maybe my egr valve is bad. So i took it off and realized the the egr valve was stuck and would not move in and out like its supposed to. So i replaced it and the truck ran really good, i mean beautifully smooth, best its ever ran, but then 30 minutes later the problem came back. Now the only other thing that i can think of that works off of tempurature is the temperature switch. Could this be causing the issues? Its got spark plugs, wires, dist cap that are abiut 6 months old so thats not the problem. What about the EGR valve solenoid?

 

[Schurkey]

Connect a scan tool, find out what the computer is trying to tell you. "Codes" may be helpful, but the real knowledge is in the data stream.

What is your fuel pressure when cold, and when warm enough to have problems?

Is the gas tank properly vented? How do you know?

 

[JeremyNH]

Maybe a bad ECT sensor. I would more suspect wiring. The TPS and ECT sensor share a common sensor ground so either or both may be erratic if the connection is poor. On wiring it can be and commonly is intermittently erratic. But The ECT is just a two wire varistor and by itself may produce enough noise on the ground to affect TPS signal. Best thing to do would be to connect a quality scan tool with a display and see if the sensor inputs are stable and expected for the operating condition of the engine. But an ECT is short money so you wouldn't be out much if you're wrong.

Note that the EGR pintle, TFT, IAT, and MAP also share a common sensor ground but it's separate from the TPS and ECT. If you had problems there as well it would be coincidental.

 

[Marcos Beltran]

Connect a scan tool, find out what the computer is trying to tell you. "Codes" may be helpful, but the real knowledge is in the data stream.

What is your fuel pressure when cold, and when warm enough to have problems?

Is the gas tank properly vented? How do you know?

I have limited access to a scan tool but i wont be able to use it for the amount of time needed for when the symptoms start to occur. Gonna have to figure out a way to make that happen. As for fuel pressure, i really dont think its a fuel pressure problem. Wouldnt the fuel pressure be a problem from when the truck first turned on? What would cause a fuel pressure issue 30 minutes after running perfectly fine, a failing pump? ive never experienced a fuel pump run for so long, run so well only to fall on its face like that. Is that even possible, well i guess anythings possible. What do you mean if the tank is properly vented? The truck is stock.

 

[Marcos Beltran]

Maybe a bad ECT sensor. I would more suspect wiring. The TPS and ECT sensor share a common sensor ground so either or both may be erratic if the connection is poor. On wiring it can be and commonly is intermittently erratic. But The ECT is just a two wire varistor and by itself may produce enough noise on the ground to affect TPS signal. Best thing to do would be to connect a quality scan tool with a display and see if the sensor inputs are stable and expected for the operating condition of the engine. But an ECT is short money so you wouldn't be out much if you're wrong.

Note that the EGR pintle, TFT, IAT, and MAP also share a common sensor ground but it's separate from the TPS and ECT. If you had problems there as well it would be coincidental.

Like mentioned in another reply i have access to a scanner but limited in how long i can use it. Been searching the internet and cant find any OBD1 scanner that dont cost an arm and leg. The ECT sensor, is that the one in the manifold or the one on the side of the block on the drivers side? I have the one for the block already, bought it a year ago but never replaced it.

 

[JeremyNH]

Like mentioned in another reply i have access to a scanner but limited in how long i can use it. Been searching the internet and cant find any OBD1 scanner that dont cost an arm and leg. The ECT sensor, is that the one in the manifold or the one on the side of the block on the drivers side? I have the one for the block already, bought it a year ago but never replaced it.

It's the one on the manifold. The sensor on the head is only used for the temperature gage on the dash.

 

[Marcos Beltran]

It's the one on the manifold. The sensor on the head is only used for the temperature gage on the dash.

The ground for the TPS and ECT is the one that is grounded to the thermostat housing right? Gonna get the sensor tomorrow and see what happens.

 

[JeremyNH]

The ground for the TPS and ECT is the one that is grounded to the thermostat housing right? Gonna get the sensor tomorrow and see what happens.

No. That wire's simply continuity of chassis ground to an otherwise poorly grounded part (water neck). These are 5V sensors and all 5V sensors are sourced and grounded at the ECM. The two sensor grounds are spliced in the harness and run to the C3 connector pin 4 in your truck.

 

[Marcos Beltran]

No. That wire's simply continuity of chassis ground to an otherwise poorly grounded part (water neck). These are 5V sensors and all 5V sensors are sourced and grounded at the ECM. The two sensor grounds are spliced in the harness and run to the C3 connector pin 4 in your truck.

Dumb question how the heck would i check that? Use multimeter to that pin 4, one lead to the pin ,one to a ground and see if im getting a constant 5 volts.? Cant i just test it from the connector itself that goes to the ECT and TPS when truck is acting up? Or is it a more accurate reading coming from the source, well not necessarily more accurate but seeing if the ECM is putting out the right signal or if im losing power at the connection coming off the ECM.

 

[Schurkey]

I have limited access to a scan tool but i wont be able to use it for the amount of time needed for when the symptoms start to occur. Gonna have to figure out a way to make that happen.

Yup. You can accomplish more with a scan tool in fifteen minutes, than with a multimeter in four hours. Maybe eight hours.

Trying to fix a computer-controlled engine/vehicle without a scan tool is two steps short of hopeless. But some folks do get lucky.

As for fuel pressure, i really dont think its a fuel pressure problem. Wouldnt the fuel pressure be a problem from when the truck first turned on? What would cause a fuel pressure issue 30 minutes after running perfectly fine, a failing pump? ive never experienced a fuel pump run for so long, run so well only to fall on its face like that. Is that even possible, well i guess anythings possible. What do you mean if the tank is properly vented? The truck is stock.

Suppose the fuel tank vent is nearly plugged, somehow. The vent tube got disconnected, and packed-full of mud. Or the tube got crimped. Or whatever.

You run the truck, your pump moves fuel out of the tank...but since the vent is almost totally plugged, no air gets in. Eventually, you drive long enough to create a vacuum in the tank. Now the fuel pump has to fight the vacuum in order to push fuel out. Fuel pressure goes too low, truck has driveability problems. You might even see on the scan tool that the short- and long-term fuel trims go rich-command as the tank builds vacuum.

Shut it off for a few hours, maybe you have enough leakage in the vent--or at the fuel filler cap--that the vacuum relieves, and the fuel pressure returns to normal.

Likely? No. But possible. The fuel tank vents via the charcoal canister, and long tubes.

Or--yeah--a failing pump that doesn't work right when it gets warmed-up. Thus my suggestion to verify fuel pressure in order to rule-out some weird stuff.

You could drive the thing until it acts-up, then remove the fuel fill cap. If the truck immediately runs better...