Rough intermittent idle and acceleration (1 Viewer)

[93yotasr5]

Searched and searched without any answers. Here I have my 94 cruiser. Once warmed up the engine will run nice and smooth, but about every thirty seconds or so it gets really rough like it isn’t firing on all cylinders. Idles rough for approximately 10 seconds and then goes back to smooth. Does it consistent at idle.

If I drive it off it stumbles hard like a plug wire isn’t hooked up and then after about 3500rpm it smooths out. Once the gear shifts and the rpm’s drop, the roughness comes back. Was getting code 28 and 21 previously so I replaced both sensors today with Toyota units. Cleared code and the same issue is here after driving for 10 miles. I replaced every vacuum hose about 2 years ago, Intake hose is new checked it all anyways and cannot find the culprit. Any other ideas or suggestions?

I will try and post a video next.

 

[Malleus]

Idle air control valve. Pull the throttle body, clean it, and the IAC. put it all back together. The FSM has all the details.

 

[93yotasr5]

I pulled the TB off and cleaned it up. I also checked all the vacuum hoses underneath. Pulled the IAC and it wasn’t really dirty and neither was the TB.

Ran cleaner and compressed air through all the little vacuum lines as well.

I’ll put it all back together tomorrow and report back.

Thanks

 

[Malleus]

You have to test the IAC. You can't tell anything by looking at it.

 

[93yotasr5]

Just found the specs for testing it. I will check that out in a few. Thanks

 

[93yotasr5]

Tested IAC and all is within spec.
Any other suggestions? Since the TB is off I’ll be inspecting all wires and also be checking plugs and plug wires, cap/rotor.

 

[Malleus]

If you've tested the IAC and it moves in and out as advertised, and your idle isn't smooth, it's the injectors.

You need fuel, air and fire to make an engine run. The air, at idle, is primarily controlled using the IAC. If the engine runs well at high rpm, it can't be the spark (unless you have a short somewhere). That leaves the injectors. The FSM has a chart with the injector troubleshooting in it. You use a multimeter to check the injector voltage at the ECU. You need an oscilloscope to scan the function with the engine running.

I posted it last week or so; if I can find it, I'll post a link to that thread.

 

[93yotasr5]

Just put it all back together and it took a while but the issue came back. I didn’t test the function of the IAC just the OHMs. The injectors were rebuilt not many miles ago (10k). I’m going to pull off the IAC and try the function. No code is being thrown or stored.

 

[robnicko]

you may want to check the fuel pump relay (circuit opening relay).
It operates by varying the voltage so lower at idle / higher at higher loads & rpm.
It operates a bit like old points ignition and can get crusted up where the contacts are. They can be opened up and carefully cleaned where the contact is made

 

[93yotasr5]

If you've tested the IAC and it moves in and out as advertised, and your idle isn't smooth, it's the injectors.

You need fuel, air and fire to make an engine run. The air, at idle, is primarily controlled using the IAC. If the engine runs well at high rpm, it can't be the spark (unless you have a short somewhere). That leaves the injectors. The FSM has a chart with the injector troubleshooting in it. You use a multimeter to check the injector voltage at the ECU. You need an oscilloscope to scan the function with the engine running.

I posted it last week or so; if I can find it, I'll post a link to that thread.

you may want to check the fuel pump relay (circuit opening relay).
It operates by varying the voltage so lower at idle / higher at higher loads & rpm.
It operates a bit like old points ignition and can get crusted up where the contacts are. They can be opened up and carefully cleaned where the contact is made

So I removed the IAC and tested it. When power was applied the plunger did not move in or out. Instead I could feel it pulsate like it was trying to work but couldn’t.

Pretty sure this is the problem, but since I’m looking at all options I will check out the FP relay while I’m at it.

Looking into IAC replacement options now.

Thanks guys.

 

[Malleus]

So I removed the IAC and tested it. When power was applied the plunger did not move in or out. Instead I could feel it pulsate like it was trying to work but couldn’t.

Pretty sure this is the problem, but since I’m looking at all options I will check out the FP relay while I’m at it.

Looking into IAC replacement options now.

Thanks guys.

The fuel pump relay isn't the problem. It would stumble at all engine speeds if it was.

You may still be able to make the IAC work. Take it apart and clean it. Rub the inside of the stator with emery or crocus cloth and wipe it clean. This worked for me.

If this doesn't work, @slow95z had one a couple of weeks ago. PM him.

 

[slow95z]

I have several used ones

 

[93yotasr5]

Here is a video of the idle for reference.
Working on getting a new IAC as we speak.

 

[93yotasr5]

Put in a new IAC and the idle issue seems to have been fixed but now I’m getting an intermittent misfire at idle and a CEL that comes on for a few minutes and then turns off while driving. Codes are 25/26. Both O2 sensors are brand new. Going to clean VAFM and see if that helps. Otherwise it seems to run just fine.

 

[Miilkman]

Put in a new IAC and the idle issue seems to have been fixed but now I’m getting an intermittent misfire at idle and a CEL that comes on for a few minutes and then turns off while driving. Codes are 25/26. Both O2 sensors are brand new. Going to clean VAFM and see if that helps. Otherwise it seems to run just fine.

Where'd you order your new IAC? I might need to look into replacing mine. Thanks

 

[93yotasr5]

I order

Where'd you order your new IAC? I might need to look into replacing mine. Thanks

I ordered a used one from @slow95z

Great seller and shipped it out super quick.

It solved my problem with the funky idle. Also I found out that when I put the TB back together, I had crossed up the two vacuum lines that were going to the VSV. Took a couple days of driving and CEL is off.

 

[SmokingRocks]

Well, you may have just replaced those O2 sensors but something may not be right still. The OBD keeps pointing you back to O2 issues, it has literally hit like 4 codes that reference back to O2 sensor or o2 circuit issues, so I wouldn't just disregard that. I'd spend more time checking that circuit to verify that the O2 sinewaves are making it to the ECU, you would need an oscilloscope to perform this test. The test and example waveforms can be found here: Picoscope Automotive Test Library | Lambda O2 Sensor Waveform Test

If you don't have an oscilloscope this is the prime time to add another diagnostic tool to your kit, either get a decent ($80-$150) handheld unit or do what I did and get a Picoscope 4402a kit that you run through your computer.

Also, don't mess with the AFM it's not like a MAF and there really isn't much you can "clean" without getting to a point where you seriously risk irreparably damaging the device.

Also Also the IAC is a stepper motor so you don't apply contestant voltage to move it instead you provide pulses of voltage rapidly, see the IAC valve inspection bench test in the FSM (EG-294). You should get my most recent revision to the 93-94 FSM from the link in my desc its well worth the free download

 

[SmokingRocks]

This is what the signal the ECU gets from good O2 Sensors looks like, I captured this image when I was scoping my 85' e30 tracing down a hesitation issue.

 

[Miilkman]

I order
I ordered a used one from @slow95z

Great seller and shipped it out super quick.

It solved my problem with the funky idle. Also I found out that when I put the TB back together, I had crossed up the two vacuum lines that were going to the VSV. Took a couple days of driving and CEL is off.

Dope Thank you! He had some spares to sell. Hopefully this fixes my issue.

 

[93yotasr5]

So the issue showed it’s ugly little head again.
While the idle was chugging, I wiggled the harness to the IAC and it stopped it momentarily. I removed the covering to the harness and it looks like some of the wires are exposed. I checked them all with a meter but couldn’t pinpoint any exact wire that was having issues. I ordered a new pigtail and will splice the wires to the new connector and see if that solves the damn issue.