running a little rough

[peejay76]

That plenum has to go down as one of the worst engineering marvels of the 20th century.

 

As to your IAC valve, I think that's the one that they say to have a replacement gasket ready, if you find it needs disassembling and cleaning. Good luck, thanks for updating here!

[Slartibartfast]

I'd be surprised if the idle controls made it misfire like that. A vacuum leak, maybe, but the idle air control pulls through the same maf as the rest of the air going into the engine. If it was stuck, you'd get a high or a low idle, but it shouldn't be screwing up the mixture, unless it was borked bad enough to suck air air from the engine bay.

 

Have you checked the rest of the plugs? Might tell you which cylinder is acting up.

[RF600]

16 hours ago, Slartibartfast said:

I'd be surprised if the idle controls made it misfire like that. A vacuum leak, maybe, but the idle air control pulls through the same maf as the rest of the air going into the engine. If it was stuck, you'd get a high or a low idle, but it shouldn't be screwing up the mixture, unless it was borked bad enough to suck air air from the engine bay.

 

Have you checked the rest of the plugs? Might tell you which cylinder is acting up.

The last time I pulled the plugs they were all white. Not wet. So lean fuel mixture and hot cylinder temps.

 

The iac seems to be operating properly. I'm leaning towards the pulse air valve, or whatever it's called. It ohm's out good. I pulled it apart to look and see what is in there. Since I can't get a picture that will fit under the 64kb, I will try and explain it.

 

There is a plate inside that is spring loaded. The plate has a slot that will open or close depending on the coolant temp, my assumption because the coolant line runs through a plate that is mounted to the plenum, then this valve is mounted to that. I tried putting it in the freezer and the plate didn't move. I tried heating it up and the plate didn't move. At this point I believe that this pulse valve is not operating like I think it should. The plate is stuck on the almost closed position. I assume when it's hot the plate is supposed to be in the open position, allowing air to pass through it. I'm half tempted to make it so the valve is always open to see what happens. 

 

I'm guessing at this point. I don't really have any other explanation of what could be happening. No vacuum leaks. I smoked the system.

 

 

 

[Slartibartfast]

EF&EC-14 explains how the idle control components work. The plate in the IACV (the tall one that sticks up) opens when the engine is cold to raise the idle speed. There's a heater inside (that's why it's got the electrical connector) which closes it over time. If it was bad, you would either have a high idle all the time (stuck open), or no high idle when cold (stuck shut). I don't know how much it should open and close from external heating/cooling (surely the heater inside is more effective), but, yeah, if yours is mostly closed at room temp with no power going to it, that ain't right. I'd expect it to be wide open when cold unless it's gunked up or the bimetal that moves the shutter is broken.

 

The IACV-AAC (the one built into the block with the idle adjust screw) is the one that's pulsed by the computer to do the fine-tuning.

 

I'm not sure why there are two valves for the same function. Maybe it was easier to add another valve than it was to add another function to the computer.

[RF600]

19 hours ago, Slartibartfast said:

EF&EC-14 explains how the idle control components work. The plate in the IACV (the tall one that sticks up) opens when the engine is cold to raise the idle speed. There's a heater inside (that's why it's got the electrical connector) which closes it over time. If it was bad, you would either have a high idle all the time (stuck open), or no high idle when cold (stuck shut). I don't know how much it should open and close from external heating/cooling (surely the heater inside is more effective), but, yeah, if yours is mostly closed at room temp with no power going to it, that ain't right. I'd expect it to be wide open when cold unless it's gunked up or the bimetal that moves the shutter is broken.

 

The IACV-AAC (the one built into the block with the idle adjust screw) is the one that's pulsed by the computer to do the fine-tuning.

 

I'm not sure why there are two valves for the same function. Maybe it was easier to add another valve than it was to add another function to the computer.

The plate on the inside has about a 40 degree open to close. It's about half way open currently. It operates freely. No binding, no signs of any soot or anything else.

 

Would you expect the idle to suffer with it stuck half way? Most of the time when I start it cold it will jump up to high idle, but drops pretty quickly. If it doesn't go to high idle, it will struggle to idle properly.

[Slartibartfast]

It does make sense that a low idle when the engine is cold would be giving you issues. The high idle is there for a reason.

 

But I assume you didn't smog it cold, and I would expect it to clear up as the engine warms up. It's supposed to come off high idle once it's warmed up anyway. Maybe the computer could be doing something weird if it's letting little or no air through the AAC due to the IACV being stuck open, with the idle still above its target? Maybe someone somewhere down the line meddled with the idle screw to try and mask the bad IACV, and that's making it worse? I still wouldn't expect that to manifest as a misfire under any condition but cold idle.

In any case, it does sound like you've found something wrong. Absent any other smoking gun, might as well chase that one, I guess.

 

I have an IACV on a spare plenum. I have no idea if it's good, but if you want I can pop it open and see if it's how you describe yours or not.

[RF600]

On 5/2/2026 at 3:27 AM, Slartibartfast said:

It does make sense that a low idle when the engine is cold would be giving you issues. The high idle is there for a reason.

 

But I assume you didn't smog it cold, and I would expect it to clear up as the engine warms up. It's supposed to come off high idle once it's warmed up anyway. Maybe the computer could be doing something weird if it's letting little or no air through the AAC due to the IACV being stuck open, with the idle still above its target? Maybe someone somewhere down the line meddled with the idle screw to try and mask the bad IACV, and that's making it worse? I still wouldn't expect that to manifest as a misfire under any condition but cold idle.

In any case, it does sound like you've found something wrong. Absent any other smoking gun, might as well chase that one, I guess.

 

I have an IACV on a spare plenum. I have no idea if it's good, but if you want I can pop it open and see if it's how you describe yours or not.

If you have the spare time to do that, I would appreciate it.

 

I don't really know why it's running as poorly as it does. I agree, it doesn't make sense. Fresh tune up, no vacuum leaks, no codes, not really anything points to why it runs like it does. 

 

Since I haven't really haven't much with the injectors, I guess they might be worth looking into? 

[Slartibartfast]

My spare IACV looks about like how you described yours. Half open at room temp. I connected 12v across the heater, and ~5 minutes later, it closed up. Left is cold, right is after powering the heater. I didn't try the freezer test, but the cutout does look like it should open a lot more than it is at room temp.

 

 

Injectors would make sense. I've got one or two marginal ones on mine. One crapped out entirely once, consistent misfire, tested open circuit, then came around the next day and worked like nothing happened. It's been working since, so I haven't gotten around to replacing it. But I have noticed a little stumble from time to time, which I suspect is related. 

 

You can check all six injectors from the harness plug on the passenger's side valve cover. Of course that'll only tell you if the coils are good, not whether they're plugged up or spraying weird. But it's a start.

 

 

[RF600]

On 5/5/2026 at 12:08 AM, Slartibartfast said:

My spare IACV looks about like how you described yours. Half open at room temp. I connected 12v across the heater, and ~5 minutes later, it closed up. Left is cold, right is after powering the heater. I didn't try the freezer test, but the cutout does look like it should open a lot more than it is at room temp.

 

 

Injectors would make sense. I've got one or two marginal ones on mine. One crapped out entirely once, consistent misfire, tested open circuit, then came around the next day and worked like nothing happened. It's been working since, so I haven't gotten around to replacing it. But I have noticed a little stumble from time to time, which I suspect is related. 

 

You can check all six injectors from the harness plug on the passenger's side valve cover. Of course that'll only tell you if the coils are good, not whether they're plugged up or spraying weird. But it's a start.

 

 

Thank you for the follow up. Mine looks like yours does in the pic. I haven't tried to put 12v to it and see what happens.

 

The injectors ohm test within spec. I know that doesn't mean much. I wouldn't think the injectors would cause the up and down when holding the engine speed around 2K. I would think a sensor is malfunctioning. Something like the MAF or TPS acting up?

[RF600]

I figured I would start over and check the TPS. I didn't really check it very good the first time. I was able to get the TPS to read. The manual says it should go up to 9 ohms resistance. The best I could bet was 7.3 ohms.