Rough idle on automatic

[Gottolovem]

I have had my auto a couple of years now and it has always had a rough idle after it warms up.
I have swapped out 4 different carburetors that were perfect on a manual transmission cars.
The deal is that in an auto tranny at a stop the engine is still under slight load if I drop the transmission into neutral it runs perfect.
Any tips would be appreciated

[Mark]

The dreaded rough idle... In theory it could be just about anything. I'm pretty sure rusty fenders could even cause it somehow, but on mine, it's always been a vacuum leak issue. Even if there are no visible leaks around the hoses, the internal diaphragms of the various emissions components could be ruptured (actually, almost guaranteed in my experience). If it only happens when the engine is warm it could be the EGR vacuum modulator. Try blocking the hose to it from the TSTV and see if the idle smooths out. It could be the idle seems fine when the engine is cold just because the idle speed is higher and there could be a vacuum leak somewhere at all temperatures. You could try bumping up the idle speed slightly in the meantime unless it's already well over 800 rpm. Good luck.

[TURTCEL]

That could actually be a transmission issue and not a carb or emissions system prob. The AT works off of hydraulic pressure that has to pump trans fluid through many passages in order to make the system work.

If you haven't done a trans filter and fluid change, now might be the time. Dirt clogs the filter and the pressure drops not allowing the trans to work properly as well as putting drag on the engine which can cause rough idle.When an AT has issues it can definitely make the engine run quirky. I have had this happen myself on an Impala I had many years ago.

I am not totally familiar with the AT Terc 4x4 trans but if it is vacuum actuated then I would look for a vacuum leak as well to be the culprit.

Hopefully that is all it is. Good luck.

Peace

[dlb]

i like mark's thought on the EGR, and if the EGR valve is sticking open due to carbon build up it will stay open all the time. easiest way to disable the system that i can think of is to remove the banjo-style bolt from the front of the engine, the one that connect the metal EGR tube to the exhaust manifold. replace it with a regular bolt, ta da. no more exhaust gases exiting the manifold. then the valve can do whatever it wants and since it's a closed system, it doesn't matter. i could be wrong but i believe that's it.

[Gottolovem]

EGR makes since I had a terrible time on the smog check this year ended up putting a new cat on to get it to pass.

I doubt its transmission trouble flushed it completely a year ago.

Thanks for the input
Can't I just disable it from the vac lines to trouble shoot the problem.

[Mark]

The automatic transmission shifting is controlled by a manual valve actuated by a mechanical linkage off the throttle. The fluid pressure is controlled by a centrifugally-actuated valve internal to the transmission. There are no vacuum lines going to the transmission. The only way I can see the transmission affecting idle is if something the engine is driving in the transmission (torque converter or gear pump) is causing too much resistance. Usually wear in the transmission will cause the opposite (slipping torque converter or pump) . As long as the transmission is operating properly during normal driving, I can't see it having anything to do with the idle problem. To rule out vacuum issues, I'd pull and plug the hoses going to the various diaphragm-actuated components (usually all the things that look like UFOs) one by one until the idle smooths out. Re: Idle speed, the autos should be at 800 rpm, VS the standards which are set at 600 I think. Maybe yours is just too low.
By the way, a few days ago mine just started having a rough/low idle speed when it is cold. It stalls out when I put it in drive unless I put my foot on the gas slightly. I assume it's a vacuum hose that might have come loose. It happens to me occasionally. I'll take off the air filter housing and have a look tomorrow when it's light out.

[dlb]

EGR makes since I had a terrible time on the smog check this year ended up putting a new cat on to get it to pass...Can't I just disable it from the vac lines to trouble shoot the problem.

no, if the valve is worn and not sealing or chunks of carbon are holding the valve open, disconnecting vacuum from the valve won't help since it will still be open. removing the valve or stopping the flow of exhaust gas from the valve is the only way to prevent exhaust gas from leaking into the manifold.

swapping the bolt is no big deal. just spray the banjo bolt with penetrating oil, let it sit, remove it, and bring it to a hardware shop to get a regular bolt of the same size. only cost me a couple bucks and now i keep one on hand so i can test EGRs easily in the future.

[ARCHINSTL]

.swapping the bolt is no big deal. just spray the banjo bolt with penetrating oil, let it sit, remove it, and bring it to a hardware shop to get a regular bolt of the same size. only cost me a couple bucks and now i keep one on hand so i can test EGRs easily in the future.

Just get a bolt sized 20x1.50mm - and also use some shallow washers when installing in the manifold threaded hole.
This is an ordinary bolt - nothing special. This also provides a tad more room around the engine.
Note that some have said that a trans drain plug will also fit.
Tom M.

[Mark]

EGR makes since I had a terrible time on the smog check this year ended up putting a new cat on to get it to pass...Can't I just disable it from the vac lines to trouble shoot the problem.

no, if the valve is worn and not sealing or chunks of carbon are holding the valve open, disconnecting vacuum from the valve won't help since it will still be open. removing the valve or stopping the flow of exhaust gas from the valve is the only way to prevent exhaust gas from leaking into the manifold.

swapping the bolt is no big deal. just spray the banjo bolt with penetrating oil, let it sit, remove it, and bring it to a hardware shop to get a regular bolt of the same size. only cost me a couple bucks and now i keep one on hand so i can test EGRs easily in the future.

On 2 vehicles I've had including the one I have now, the egr valve itself was fine (as far as I know). Both times it was the egr modulator, which is a plastic UFO-thing with a diaphragm in it that opens or closes to send vacuum to the egr valve diaphram. There's a hose from the TSTV that goes to this modulator. I forget which nipple on the TSTV is for EGR, but if you unplug it and block it off, you can rule out (or in) a vacuum leak in the egr modulator pretty quickly.

vacuum.jpg

[tercel4wdrules]

I believe plugging the 'M' port on the TVSV will disable the EGR.

[Gottolovem]

Thanks for all the input I should have some sort of results later today.

[Petros]

usually an issue like this is vacuum related, a vac operated device that is bad will act like a vac leak too, not just a leaky vac line.

OTOH, several times I have seen weak spark from the distributor act like this too, at higher reves it put out better spark, but at idle it is weak and allows it to stall out.

So also verify you have reliable spark as well as checking the vac operated equipment.

[Gottolovem]

Well everything checks out removed and went through every bit of EGR and it is fine.
Double checked every diaphragm as per FSM and it is all good.
The car runs awesome I guess I'm being anal about a slightly rough idle that others may not even notice.

Thanks again for all the feedback.