Erratic idling

[bigalpha]

Yeah, I replaced both the IAC and TPS with new (properly calibrated) and it made no difference.

There is also no rattling coming from the cat and nothing to lead me to think it's clogged/broken.

Yes, every time the vacuum gauge would spike, it corresponded to an increase in the RPMs. The RPMs would spike, then a fraction of a second later, the vacuum needle would spike.

The truck drives fine and I don't notice any issues with it. HOWEVER, when the engine is cold and I start it up, I can hit the gas and there is a momentary delay of nothing, then the engine revvs. It almost sounds like it bogs down then catches up to itself. Then there is a popping sound coming from the engine bay. I can't tell where it's location is. I didn't notice this until I was fooling around yesterday with the vacuum gauge.

 

[Ecomike]

OK I just got a look at this. I don't see what I call needle vibration. You vacuum looks good to me. What I see is a hunting idle speed problem. If this was a Renix I would say you have a wandering ground or bad TPS wire connection somewhere. Get an analog volt meter on the TPS wires, one at time versus the battery negative ground. That should locate a wandering signal source. Typically the ground. I see from your bio you have a 95 right?

In you case I would start checking the engine and frame ground wires, sensors grounds. Then start looking for a sensor that has a wandering output, like the MAT and CTS and TPS. MAP will wander as rpm wanders so no sense in testing it. It could even be the ECU.

 

[bigalpha]

This is a RENIX. It's an 88, 4.0, 6cl.

I've replaced the main batt to block ground with a 5-90 cable. I also cleaned the firewall ground and the one behind the coolant temp. sender.

Tested the grounds to the batt neg terminal and they are all below .3 ohms.

 

[Ecomike]

Yeah, I replaced both the IAC and TPS with new (properly calibrated) and it made no difference.

There is also no rattling coming from the cat and nothing to lead me to think it's clogged/broken.

Yes, every time the vacuum gauge would spike, it corresponded to an increase in the RPMs. The RPMs would spike, then a fraction of a second later, the vacuum needle would spike.

The truck drives fine and I don't notice any issues with it. HOWEVER, when the engine is cold and I start it up, I can hit the gas and there is a momentary delay of nothing, then the engine revvs. It almost sounds like it bogs down then catches up to itself. Then there is a popping sound coming from the engine bay. I can't tell where it's location is. I didn't notice this until I was fooling around yesterday with the vacuum gauge.

Is the map sensor vacuum line connected properly to the throttle body?

What do you mean properly calibrated? Is this a 95 or Renix?

Your last comments still sound like a bad TPS, so maybe the wiring from the TPS to ground & the ECU is bad somewhere. or shorting somewhere.`

 

[bigalpha]

It's a RENIX. It's a 1988.

The TPS is set at .8V.

Yes, I think the MAP line is hooked up properly. When I disco the line, the engine stalls out. I also replaced it with another from a JY with no change.

Where does the TPS ground at?

 

[Ecomike]

This is a RENIX. It's an 88, 4.0, 6cl.

I've replaced the main batt to block ground with a 5-90 cable. I also cleaned the firewall ground and the one behind the coolant temp. sender.

Tested the grounds to the batt neg terminal and they are all below .3 ohms.

OK, then try what I said on the live engine meter voltage watch of each sensor. Test the grounds while hot, on volts, running, for spikes in back voltage between the sensor ground wire and battery negative post. Test the MAT sensor (manifold air) for wandering values. Sometimes a wire is broken inside the insulation, and making poor intermittent connection with itself. It shows up when you wiggle the harness wires and from engine vibration. Does your 88 have the C-101 connector? I think it was only 87?
You may need to check the ECU pin out readings to see if the TPS signal is getting to the ECU, but do that last, as that is a rare thing.

 

[Ecomike]

It's a RENIX. It's a 1988.

The TPS is set at .8V.

Yes, I think the MAP line is hooked up properly. When I disco the line, the engine stalls out. I also replaced it with another from a JY with no change.

Where does the TPS ground at?

I had to run new grounds for both sides of my TPS sensor directly to the battery. It solved some idle and tranny rpm shift problems. I did not bother to try and trace them through the firewall in multiple passes to the ECU and battery, I just ran new ones right from the TPS sensor to the battery. I also had to run a new ground from my TCU computer to a frame ground nearby.

 

[bigalpha]

So let me get this straight.

Hook up all the sensors and test them for wandering voltage as the truck is running? So I would test each line in the TPS to the Neg Batt terminal, hooked up as the truck is hot and running?

The same goes for the grounds and MAT sensor?

I am not sure of the C-101 connector nor the pin outs for the ECU. I'll have to research those. I almost want to go ahead and run a new ground from the TPS to the Battery, but I hesitate cutting wires and then having to put them back together.

I suppose I can also test the ground on the TCU and make sure that it's grounding properly. Though, I have no reason to suspect otherwise as the trans still shifts just fine. Also, there is a high idle while in D, and it does not hunt as if it was in P or N.

 

[bigalpha]

I, in fact, DO HAVE the C-101 connector. It's a monster of a wire bundle. I will look more into problems with it and how to correct them.

Still unsure of the ECU pin out. Still looking.

Should have picked up a junkyard TPS so I could splice into it.

 

[Ecomike]

So let me get this straight.

Hook up all the sensors and test them for wandering voltage as the truck is running? So I would test each line in the TPS to the Neg Batt terminal, hooked up as the truck is hot and running?

The same goes for the grounds and MAT sensor?

I am not sure of the C-101 connector nor the pin outs for the ECU. I'll have to research those. I almost want to go ahead and run a new ground from the TPS to the Battery, but I hesitate cutting wires and then having to put them back together.

I suppose I can also test the ground on the TCU and make sure that it's grounding properly. Though, I have no reason to suspect otherwise as the trans still shifts just fine. Also, there is a high idle while in D, and it does not hunt as if it was in P or N.

To be precise, engine running, in idle, back probe one sensor and one sensor wire at a time, the second probe tip (or alligator clip) solidly grounded to the negative post of the battery, and look for wandering voltages with an analog (needle) volt meter. Hot, cold, does no matter, but do it while the rpms is wandering up and down, just like your vacuum was wandering.

The wandering, hunting idle always disappears in drive, and just shows up as a high idle. If all else fails, try running with out the O2 sensor connected to see if it is causing the problem.

I have left my C-101 alone, except to clean the metal connections with q-tips and solvent. I left the black tar like sealant alone. I gust ran new wires where I needed to. Mine had a power, and very low rpm up shift problem. Turned out the +12 volts for the torque converter lock up, and ground to the TCU were bad. I only fixed what was bad. I was afraid to go all postal on the C-101, LOL.

 

[bigalpha]

Excellent, I'll stop and get an analog meter for this. HF has them for cheap. Maybe it's something weird like a broken wire in a place that I cannot see.

I will also try unplugging the O2 sensor and see what does. I get pretty terrible mileage (13mpg) so I wonder if the O2 sensor is bad. I wasn't aware a bad 02 sensor can cause crazy idling.

Thanks for the help!!

 

[Ecomike]

Excellent, I'll stop and get an analog meter for this. HF has them for cheap. Maybe it's something weird like a broken wire in a place that I cannot see.

I will also try unplugging the O2 sensor and see what does. I get pretty terrible mileage (13mpg) so I wonder if the O2 sensor is bad. I wasn't aware a bad 02 sensor can cause crazy idling.

Thanks for the help!!

I have seen a few reports of a marginal O2 sensor doing odd stuff like that. I suspect they had a moving short of some sort internally that the ECU read as data, making the computer hunt.

 

[bigalpha]

UPDATE!!

I unplugged the O2 sensor and it didn't make a difference. I went ahead and placed a new ground wire from the TPS (3 prong connector) to the Batt-.

It's basically solved the erratic idle problem! It still isn't perfectly smooth, but it's around normal.

I must not have done a good job when I tested the ground against the battery.

 

[Ecomike]

UPDATE!!

I unplugged the O2 sensor and it didn't make a difference. I went ahead and placed a new ground wire from the TPS (3 prong connector) to the Batt-.

It's basically solved the erratic idle problem! It still isn't perfectly smooth, but it's around normal.

I must not have done a good job when I tested the ground against the battery.

Or it had a wandering ground, reads good one minute, then not the next.

Sounds like you figured out where the gremlin was! Congrats. Don't forget to reconnect the O2 for gas mileage.

 

[bigalpha]

Or it had a wandering ground, reads good one minute, then not the next.

Sounds like you figured out where the gremlin was! Congrats. Don't forget to reconnect the O2 for gas mileage.

Something -- I'll go ahead and leave the ground I ran in place. No point in trying to track it down otherwise.

I had poor mileage already. I think it was the o2 sensor.