YACT: Strange engine running prob. How do you diagnose?

[Mark R]

Hyundai Lantra 2.0 DOHC 1999

Recently developed a terrible problem with hunting idling: Engine would rev up to 1500-1600 rpm, before dropping down to about 400 rpm (occasionally stalling), before revving up to 1500 rpm again.

Problem dissappeared when accelerating.

Dealership diagnosed as faulty throttle position sensor - and replaced it.

Problem is better but still remains.

No more unstable natural idle but:

Unstable acceleration - very little acceleration for first few mm of pedal travel, then a surge of acceleration. Normal acceleration the rest of the way.

Unstable overrun - when the car is slowing down with the accelerator released, when the engine speed drops to 1500 rpm, the engine will accelerate then slow down repeatedly, until the car slows down a bit more.

Any ideas? The car already also just had the ICSV replaced (which didn't help either).

 

[DaTT]

Sound like what was replaced...the TPS. When they replaced it, did the "re-flash" the computer? Or they just put a faulty one in. Could also be the MAF meter?

 

[StevenMara]

I think when putting a new sensor in you just have to unplug the battery to reset the computer. At least on Saturns.

I don't know if this would help, but have you cleaned your throttle body?

 

[CrackRabbit]

My first thoughts were MAF/IAC, could also be a broken vac line, or gasket. Worse case would be a cracked intake manifold.

 

[iamwiz82]

MAF, TPS, CPS

 

[BobDaMenkey]

I'm thinking it's a vacum leak. My old BMW was notorious for this, when we pulled the engine we found out why. Someone had patched the vacum lines with ELETRICAL TAPE.

They can be a pain to track down properly though.

 

[Ornery]

1. FINDING & FIXING VACUUM LEAKS

 

[Mark R]

Thanks for the suggestions.

I'm beginning to think that the new TPS isn't that great.

Testing it out today, the transmission has been doing a lot of random kickdowns since the TPS was replaced. Indeed, it got so bad just now, that the transmission locked out into 'get you home mode'

However, this info might be useful. I tried running without the TPS. (I've done this before in several other cars, and in most cases you don't notice it).

Without the TPS, the car is totally undriveable. When the engine gets to about 2000 rpm, the engine slows down (fuel injectors get shut off) until it virtually stalls, then it starts pulling again, over and over. So, I'm guessing that something else is up.

I don't have the eqpt to test the MAP sensor (no MAF on this model).

Thanks for the info on vac leaks, will look into it.

 

[SuperSix]

Originally posted by: CrackRabbit
My first thoughts were MAF/IAC, could also be a broken vac line, or gasket. Worse case would be a cracked intake manifold.

What he said.. and the acceleration issue sound like the dealer didn't adjust the TPS correctly.

 

[Ornery]

1. Update?