Idling too fast **** Fixed it!****

P

Peter Mahr

Daily Driver
With these forums help, I have built a 1500 FI engine out of 2 donor cars, a 1981 and a 1982 X that I purchased from Trent McKinley 8 years ago. After installing the engine in my 1974 X, it started on the first try once I got oil pressure and primed the fuel system. It would not idle slower than 1200 rpm but as I was so happy to be on the road, I temporarily disregarded that issue. Now, 200 miles later, I’m back at it. My first concern was that the throttle plate was not totally closing since, with the idle stop screw totally backed off and the Idle speed adjustment screw totally in, the idle was still at 900 rpm. I disconnected and plugged the auxiliary air regulator, pinched off the distributor vacuum line and the carbon canister vacuum line, to no avail. I tried to re-center the throttle plate by reaching into the air horn, loosening the 2 screws and manipulating the throttle shaft. I’ve changed out the air flow sensor since my original one was missing the plug on the mixture adjustment screw, all to no avail.

The engine starts on the first try and pulls hard, but I would like it to idle correctly.

In the last 50 miles, my tachometer has also started to indicate erratically, mostly staying at about 4000 rpm, but I think that is another issue.

What do I do next? Shouldn’t the engine die with the idle stop screw backed off, the idle speed screw all the way in and the aux air regulator plugged? What am I missing?

Thanks for your help.

**** I checked for air leaks with propane - found none. I re-centered the throttle plate and re-cleaned the plemb snout with carb cleaner and wa-la, the throttle plate closed further making the idle stop screw functional again. Readjusted the idle speed screw and the TPS and everything works as it should.****
 
Last edited:
Vacuum leak somewhere? Tony would say turn on a propane tank unlit to see if you get a surge in idle. I use carb cleaner or similar but it's messier.
 
First off, you cannot go by the tach in the instrument panel. Typically they are accurate enough to get a general idea of engine RPMs but they do not read accurately enough to set the idle.

For example, on my stock '86, my idle is set for 900 but the tach reads 1050-1125RPM. My measuring device was a thousand dollar Fluke automotive multimeter borrowed from a mechanic buddy.

The simple answer to your question is "false air," which is the Bosch FI diagnostic term for air that has not been measured by the AFM that is allowed to enter the intake system. When chasing this demon, you must assume that every component of the intake system is defective until positively proven to be good.

I chased this condition on my car for a couple of weeks until I discovered pretty much by accident that the job I did "refurbishing" the intake plenum was not done as well as it should have been....the throttle plate (which I disassembled to replace the throttle shaft bushings and seals) was not properly positioned on the throttle shaft and the plate was not fully and completely sealing against the inner circumference of the "snout" of the FI plenum.
 
Thanks Dan and Jvandyke

I was using a tach/dwell meter to measure RPM since my dash tach is behaving erratically at present.

I will attack the engine with propane tomorrow to check for air leaks!

I am leaning towards the throttle plate not sealing since on the extra plemb I have in stock, the linkage on the throttle shaft seems to return further in the closed position. I was hoping not to need to switch plembs.
 
It's good that you have another plenum assy to compare.

If you think it is the throttle plate not seating properly, first remove the TPS (throttle position switch) so that is totally out of the equation. Who knows, the TPS might be a little sticky or adjusted wrong and if you get more backward travel toward closing after removing, it could have been the culprit.

If it's not the TPS, keep it off the throttle plate, and remove the intake snorkel hose, disconnect the short linkage cable, loosen the two throttle plate screws***, and then maybe squirt a little wd40 onto the plate to help it slide in the shaft slot. Keep the springs on and repeatedly open the throttle to the max and let the springs snap it back shut. Hopefully it being loose, plus the lube and the forceful snapping back into rest position will bump the plate to a fully seated resting position.

Doing the above would of course be easier if the plenum were removed from the runners and on a bench, but it could be done in-place, too.

***These screws are usually very tight from the factory and the slots are easily mangled if you try to use too small of a screwdriver.
 
You must log in or register to reply here.
Back
Top