Stalling/RPM dip!

1987 Toyota van, manual trans, 4X4
A couple of weeks ago it was time to renew my tags and that meant taking it for emissions testing. For day-to-day use I do not have a catalytic converter installed. I have a piece of straight pipe so I had to reinstall the catalytic converter for testing. The van passed emissions just fine but I did not swap in the straight pipe right away. About that time the van began having an RPM dip and/or wanting to stall at stoplights but only when the engine is warmed to operating temperature. The RPM dip/stall was more severe the more rapidly I decelerated. Also the stall/RPM dip is more pronounced when stopping pointing downhill and less when pointed uphill. I inspected the vacuum and air hoses and they are all in good condition. I shorted the diagnostic plug and the computer gave me a code 11, throttle position sensor. I went to the junkyard, found another throttle position sensor, took it home tested that it was good with a multimeter and installed it. I removed the old TPS and it tested bad in the idle position. (on a side note these TPS’s are very serviceable! Just pop the cap off and sand the contacts!) Installed the replacement, cleared the code and went for a test drive. The damn problem still existed though it seemed a little less severe. I checked to see if the computer was giving a new code but the computer says everything is fine. Today I removed the cat and reinstalled my straight pipe and went for a test drive. Again improvement but not complete resolution. Before the problem arose, at operating temperature the van was idling at a factory perfect 700 RPM. Currently at operating temperature the van is idling at 600 RPM and at a rapid stop momentarily dips to about 400 with just a hint of a sputter. Anybody with a similar experience and/or suggestions?

I'm not completely sure about the TPS contacts, but typically modern electrical contacts have a thin coating of material that is very conductive & resists corrosion. Sanding points that no longer conduct will typically restore function (temporarily), but will also remove this coating.

I'm not sure what your problem is, but if you're overdue for a tune-up, or don't know when or if it's ever been done, I would suggest starting by replacing parts like spark plugs, plug wires, distributor cap, rotor, PCV valve, air filter, and fuel filter. If these parts were recently replaced, then disregard. How is the van for power? Does it have a miss? When you say "vacuum and air hoses are in good condition" A assume you've checked the throttle body hose to verify it hasn't started cracking........right?

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Originally Posted by timsrv

I'm not completely sure about the TPS contacts, but typically modern electrical contacts have a thin coating of material that is very conductive & resists corrosion. Sanding points that no longer conduct will typically restore function (temporarily), but will also remove this coating.

I'm not sure what your problem is, but if you're overdue for a tune-up, or don't know when or if it's ever been done, I would suggest starting by replacing parts like spark plugs, plug wires, distributor cap, rotor, PCV valve, air filter, and fuel filter. If these parts were recently replaced, then disregard. How is the van for power? Does it have a miss? When you say "vacuum and air hoses are in good condition" A assume you've checked the throttle body hose to verify it hasn't started cracking........right?

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Of the parts you have mentioned, the only unknown is fuel filter and PCV valve, they have been replaced recently but I guess I should not assume they
are good.. All the other parts are in good working order and have been checked recently.

Is the van hitting on all 4 cylinders? While it's idling, one at a time, pull spark plug wires (then put back). If you find one that doesn't affect rpm's when pulled then you have a cylinder that's not firing. Check that & report back. Tim

PS: your problem doesn't sound fuel filter or PCV valve related.

I am having a similar issue with my 1985 Le (120k). I replaced the TPS sensor and it still has idle issues. I'm thinking it is the IACV. Also, someone replace the "idle-up temperature switch". Its the one next to the temp sensor. Not sure where to find it or what the real name of it is since it does not come up in a search. Its the one with the round connector by the radiator cap.

The air valve between the two halves of the intake manifold can be tested by pinching it's air tube closed when cold & releasing. Pinching should make engine RPM drop when cold, but not when hot. I've yet to see a failed air valve on a van, but I'm sure it happens. These are expensive and a PITA to replace, so hopefully this isn't the issue. Being stuck in the open position would make the van idle high when the engine is warm, but wouldn't affect it when cold. Being stuck closed would cause low engine RPM when cold. The affect it would have would be constant (not likely to be fluctuations in idle speed unless it was randomly opening/closing........but that's not likely to be happening).

There isn't an idle-up temperature switch on the vans, but if I remember right somebody posted a picture of the sensors/switches by the radiator cap and was incorrectly calling the "pressure-up" temperature switch the "Idle-up" temp switch (it's in the front most part of the filler cap area with a single wire going to it). If this is the part you're referring to, it does not affect idle speed. TPS doesn't have much effect on idle speed either, it's more of an emission device that affects electronic advance. Tim

I'm having both problems. The idle is low when cold (600) and high when warm (1k).

I will do the pinch test. I have not tried adjusting the idle screw yet. Though this will probably make it un-drivable when cold. I'll see.

Shot in the dark here, but one of my old van's ('87 4WD auto trans) also didn't have a cat-converter. Needing to pass DEQ prompted me to do the same as you, but problems arose with the oxygen sensor when I installed the converter, resulting in it running poorly. Of course, I never figured out what was wrong but instead registered it in a different County to avoid the emissions test! But regardless, if memory serves, the O2 sensor is after the cat-converter so just throwing the Cat in is very much going to change what the sensor sees... maybe? best of luck!