First Gen surging at idle/CEL (long)

Hi all

My 1999 2wd MT 2.4l, 213K Tacoma started surging (revs rising, then falling almost to stall) at idle recently. The check engine light (CEL)also comes on intermittently, but it seems to run well otherwise. I bought the truck less than 1000 miles ago, and it passed smog for sale then, so I was bit surprised by this turn of events.

I replaced the plugs and checked for loose/rotted/cracked vacuum lines (found none) and/or obvious faulty plug HT leads. I was able to borrow a code reader and found P0120 (TPS sensor A circuit) and P0402 (Excessive Gas recirculation flow) codes. Cleared the codes, but they popped right back up.

Tested the TPS and confirmed 5V supply, metered voltage reading went from 0.644V at idle, 3.9V wide open. The voltage seemed to rise smoothly with no drop outs throughout throttle opening, and the connector/wiring appears to be solid/undamaged. I sprayed the connector/TPS socket with Doxit for good measure...no change. Are those reasonable readings?

Checked EGR by applying vacuum to the top port at idle and it promptly stumbled and died, which suggests the EGR is operating to some degree, or at least not stuck open or carbon-plugged shut.

Seems like next step is to check EGR temp sender resistance, but I can't seem to get the connector undone! Any tips here to disconnect it ? It's pretty awkward to reach, and doesn't appear to have any obvious snaps, catches or releases. Brute strength and pliers seem counterindicated here...will keep fussing with it.

I think the next step if the EGR temp sensor shows normal resistance is to check the EGR solenoid valve...where the hell is it? Under the intake manifold somewhere? Again, hints are welcome!

I should mention too that the exhaust manifold is missing the heatshield and has a hairline crack between the numbers 2 and three cylinders. A very small exhaust gas leak can be felt, and I bought a new manifold to replace it, for the obvious reason of carbon monoxide poisoning risks. Haven't installed it yet, but could the tiny leak be causing or contributing to the CEL/DTC's?

Any help or suggestions would be very much appreciated; my other project car is a 1966 Avanti II with the original Chevy 327....contact points/coil are the extent of the electronics there!
great forum!

Marc

 

Bumping this to ask if I’m too long-winded, the topic is not of general interest or what.

When posting a problem on a forum, I’ve always thought detail is good…now I’m thinking I ran on too long!

any feedback, please?

 

EGR solenoid is in the center of the intake plenum, you can kind of reach your hand in there for it. Sounds like you might have a stuck open EGR which can happen regardless of what the solenoid is up to as they get grimy. Symptoms sure sound like a stuck open EGR valve.

 

Thanks for the quick reply!

I was thinking stuck EGR as well, and thought that the engine stumbling abd dying when applying vacuum to the top port ruled out a stuck valve.

Could it be “partly stuck”, allowing some exhaust gas recirculation resulting in the surging idle (and DTC’s), and applying vacuum to the top port just opens it all the way and kills the engine?

What next? Remove inspect/clean/replace the EGR?

 

It's very possible it is stuck partially open. The temp sensor is the only way the computer can identify what the EGR is doing since it doesn't have a position sensor. The flow code is telling you there is flow when the EGR is commanded closed so that's the big clue. I'd take the valve off and see how it looks, might just need a clean. You can also cut up a piece of soda can to make a block off to sandwich in between the valve and intake to test if that clears up the running issues.

 

Excellent suggestion, especially since the EGR valance ain’t cheap! I don’t mind replacing parts when necessary, but it’s always good practice to diagnose as far as practical before throwing money at a problem. I very much appreciate the knowledge.

Will report back as to findings.

 

The front 02 sensor is VERY sensitive. Fix the exhaust leak first before you spend any more time diagnosing.

 

Got the new manifold on the bench…installing before any further work.

 

Apologies for long delay…life got in the way!

replaced the cracked exhaust manifold…no change.

Finally disconnected the Egr temp sensor plug….reads around 300k ohms cold, around 10k ohms hot. I think this means it’s ok.

Plan to remove, inspect and clean the EGR valve today.

Comments welcomed!

 

Reset the PCM and drive her and get back to us.

 

Pardon my ignorance, but what is a PCM?

 

My suggestion is to clean the IAC (Idle Air Control) on the throttle body with throttle body cleaner.

 

KdF, how does the IAC produce the excessive exhaust gas recirculation code?
And would you look there prior to removing/inspecting/cleaning/replacing the EGR valve?

By the way, the EGR vacuum control valve checks out ok…boy, that’s a monster to get to!

 

Ok, the EGR valve appears to be ok. Mild carbon/soot in the passages, but no chunks or obvious buildup/blockages. I was able to fill the inlet (from the exhaust manifold tube side) with carb cleaner, and there was no leakage until I applied vacuum to the diaphragm tube with a Mity Vac, then cleaner flowed freely past the valve. Diaphragm held steady vacuum as well.
Blasted it all out for good measure, but really quite clean, compared to what I’ve seen in other engines.

So this looks like there’s something telling the EGR to open when it shouldn’t.

Any thoughts before I put this back together?

 

This is why "I" would clean the IAC.

This is most likely the EGR problem. Who says that one can not have 2 separate problems pop up at one time?

 

The PCM is the Powertrain Control Module or the computer. It controls the ignition and fuel injection and it learns your driving habits. It stores driving information both good and bad and when you reset it is reset it to how it came from the factory, a known starting point.

 

Thanks….thought it was called ECM.

Finally got things straightened out, and thanks to all.

Removed and cleaned the throttle body and IAC… that cleared up the surging idle and stopped throwing EGR codes, but still CEL was still coming on with TPS code.

Although the original TPS seemed to test ok (see first post), I replaced it and that seemed to cure it…runs fine and no CEL’s

Again, thanks to all for the tips, expertise and education.
Marc

 

Thanks for the update. The TPS can have dropouts. There is a 5v reference signal provide by the PCM and 2 reference wires in most systems, I am going to have to re read the manual on Toyota, anyway, one return wire will start off at less than 1 volt and increase when the throttle is opened and the other will start at 5v’ish and reduce as the throttle is opened. The PCM reads both signals and calculates throttle position based on one or the other signal, kind of a backup and a verifier at the same time. If the resistor carbon track in the TPS has been worn thin because the wiper arm has wiped part of it out, that is a drop out. The wiper arm spends 90% in the first 1/3 of the sweep because that where your have the throttle most of the time… those little adjustments to maintain your speed.