1000 to 2000rpm surging idle problem

[StacytheBloke]

Just started my 93 LS400 engine on the bench, it has been stood for just under a year now, (i never heard it run before so i dont know whether this problem existed before but it was from a well crashed car)

The engine starts straight up and revs fine but the idle is irratic. It cycles from about 1000 to about 2000 rpm (1 second frequency)

I think the Idle speed control valve may be playing up, anyone got any other suggestions r any ways of testing it.

Btw a 1uz with just headers and no more exhaust system is bloody loud!

The engine does have a sliced loom (cut and rejoined at the bulkhead by me) but i did check all my wiring and it appears to be ok. The other thing is i have removed the egr system including the vsv valves. (could this be causing the problem?

Thanks in advance
Stacy Johnson

 

[mshawari1]

I had similar to that problem before but after I install complete exhaust the problem is cleared !!​

 

[StacytheBloke]

Ahh, since i am running on only headers it sounds like it could be the problem. Will make up an exhaust and try again.

I wonder if it is because the oxygen sensor is so close to the end of the exhaust? just speculation.

Thanks for that.

 

[Peewee]

Normally idle hunting is caused by an intake leak after the AFM

 

[StacytheBloke]

Yeh thought it could be a leak at first but checked it all. but this is quite extreme and very consistently cyclic, will video it at the weekend to really confuse people. That said i am sure someone has experienced the problem before.

I have blocked all the intake lines i am not using and also the brake booster pipe. I did read on one of the other lex sites that someone experienced the exact same problem due to a failed idle speed control valve. (was tagged onto the article on rebuilding it wherever that was)

 

[Peewee]

I've had leaks cause extreme hunting before.

Basically what happens is that while the TPS is in IDL mode, the engine is limited to about 2500rpm. If the rpm reaches that level, then the ECU shuts down every 2nd injection until the rpm comes down again, and then the injectors start firing again, and the cycle begins again.

The rpm's go up because of extra air getting in to the motor, usually via a leak.
Sometimes though some people have incorrectly installed the TPS, and jammed the throttle open.

Quick test is to unplug the TPS and see what the revs do.
If they climb very high, its most likely a leak, or a dead ISCV.
If they settle down, then... I don't know...

 

[StacytheBloke]

Thought i'd better post how i solved this issue. Since my car had a cut loom at the bulkhead i had to re attach all the wires- unfortunately i had two yellow wires backwards, one to the idle speed control valve and on to something else. Swapped them back and problem solved.