Dhobbit

I'm SCREAMING out for HELP: checked forums etc. re high idle problem.
About 2 weeks ago my 1992 4runner 3vze engine with petrol and lpg gas conversion began idle at 1500rpm.running petrol or gas same problem. Starts from cold and idles at about 1000rpm until engine fully warmed up after run, now 1500rpm. I can bring this down to 900rpm with idle speed adjusting screw fully in but then get poor starting with cold engine. No fault check codes. Checked air volume unit ok, checked throttle sensor ok, checked dashpot ok, checked timing ok, checked engine coolant temp, sensor ok, checked cold start injector system ok, checked for vacuum leaks seems ok. Noticed that by allowing more air directly into throttle intake idle speed drops, suspected air volume flow unit but it checks out ok, anyone got an idea on where to go from here. thanks for looking.

TNRabbit

Idle speed screw shouldn't have ANY effect on the cold starting....your ARE adjusting the inset flathead screw on the front (grill side) of the plenum, right?

Dhobbit

Hi and thanks for quick response, yes I'm adjusting the flat/horizontal idle screw which when fully in to slow high idle hot engine seems to effect the cold engine starting-have to kick engine over a few times to start- kicks over ok, cold engine, when idle screw backed out to original position.

TNRabbit

If you have to screw ALL the way in to bring your idle down, you have a vacuum leak, or your TB isn't closing fully.

Dhobbit

Throttle checked and unit cleaned, will check again for vacuum leak and post results, thanks again

TNRabbit

You can't see vacuum leaks; I found using an unlit propane torch works great. Listen for idle up.

Dhobbit

Tried it with carbie cleaner, hope to get hold of vacuum gauge to test vacuum lines tomorrow, also as you suggested will go over it again. Thanks

DupermanDave

I've been having an idle issue for a while. I've fixed it for the most part, but in the process of finding what's wrong, I also brought up another problem. My main problem was the throttle body bushing/bearing had way too much play in it and the butterfly valve wasn't seating properly. I've since fixed that, but in the process of tearing down my engine to find a problem, I brought fourth a vacuum leak.

I was searching and searching for my vacuum leak, using water and propane to no avail. One day, I heard the hissing noise under my hood, and knowing all my gaskets were fine (new) and the vacuum lines were good, I couldnt narrow down where the hissing was. So I got my bottle of soapy water I had been using to clean my rims off before I painted them, and I just dumped it all over the intake where I heard the hissing. It started to idle lower and lower and the soapy water started bubbling and spurting. I had found the leak. It's on the NEW intake manifold gasket.

Long story short, keep checking and keep checking for a vacuum leak. I had checked it 3 or 4 times before I just dumped a bucket of water over it. This last time I found it. If it sounds like a vacuum leak, it has to be a vacuum leak.

TNRabbit

Just remember you have vacuum hoses going all the way down to the front diff...

Dhobbit

Thanks DupermanDave looks like both of you are thinking Vacuum, out to do a check again, noticed only a few hours ago that just maybe I'm loosing a tiny bit of coolant as the engine was overhauled last year after head gasket blew, maybe somewhere around the manifold is the problem. will post back with results.

Dhobbit

Hi, rechecked vacuum again and again then I pulled off the throttle assembly and checked the mechanical bypass valve which seems to be operated by the water coolant flowing through the throttle unit. I put the throttle body with the mechanical valve part into water and heated it up, the valve is open when engine cold and closed when engine hot. I found that it was not fully closed so small amount of air was going through throttle bypass. I screw the valve spring down 2 turns to get a better seal and refitted the throttle unit. Engine now running as normal with idle set at 850rpm-will drop to 250rpm if idle speed screw is fully in-. Hope this is the fix for my high idle problem. Thanks to all for your help, it made me keep on looking.

blue_CA_yota

Where is this mechanical bypass valve that was stuck open? I've got a high idle issue, and even with my idle speed screw adjusted all the way shut, the lowest I can get my idle is about 1000rpm.

Dhobbit

Hi, remove the throttle body and behind the throttle sensor unit there should be a plate remove the 4 screws holding the plate and gasket, you should see the gel activated valve. Check its centre is recessed when cold and out when hot. This method allows air to bypass the throttle butterfly when the engine is cold, maybe its stuck in this position. remember this is for 3vze engine

blue_CA_yota

Ahh, missed that important info in the first post.. Thank you anyways, I'll look to see if I've got a similar butterfly valve in my tb..

just to clarify, this valve is BEHIND the throttle plate? Thanks again..

Dhobbit

Its behind the throttle sensor unit (held on by 2 adjusting screws) which is attached to the butterfly on the 3l 3vze engine.