At wit's end with high idle

[mhackney]

I have been trying to diagnose my 70 S's high (1600rpm) idle for what seems like ages. Everytime I discover something that I think "aha, that's it!" I turn out to be wrong! Case in point: Of Mice & MFI where I discovered a dead mouse in the hose running up to the MFI thermostat. Surely that would be the problem! But no, the idle speed is still high and the thermostat seems to be working fine. I measure the temperature on the thermostat housing at 150 degrees F when the engine was warmed up to 180.

I've replaced air filter, oil, oil filter, plugs, points, set dwell, checked throttle and mfi linkages (I had a problem there also where 1 linkage was too long and was preventing the throttle from closing all the way), adjusted valves, checked throttle air flow. I even did a Q&D advance curve which looks reasonable - except that I was not able to go down to 950rpms.

The good news is I've found and corrected a lot of little problems. But, I still have not solved the fast idle problem. The engine responds well when driving. Sudden decellerations sometimes results in some "rumbling but no back fires. I have not touched the MFI. I have read and tried to follow the Check, Measure, Adjust proceedure.

I really need some guidance on how to proceed before I go crazy!

Cheers,
Michael

[tcesar888]

Hi Michael,

I have a '78 SC. I had similar problems with my idle (more so upon warm-up). It was narrowed down to a faulty port on my WUR (Warm Up Regulator). Honestly I don't know if your model year has one...? Anyway, it may be something to look into; its easy enough to test... Hope this helps.

WUR Issues...

Good luck!

Tom

[sig_a]

Seems like you checked all there is to check, but perhaps thoroughly cleaning, lubricating and adjusting throttle linkage cable and bell crank where, I presume, it attaches to the driver side of the gearbox. In my case, the high idle would be temporarily solved by slightly pulling back on the top of the accelerator pedal. As a result of this temporary fix/diagnosis, I disassembled, cleaned and adjusted the entire linkage system from where it exits the tunnel to where it reattaches to the bell crank on the side of the transmission and then beyond that to the linkage leading thru and to the engine compartment. It took two attempts and has been fixed for months. Hope this helps.

[mhackney]

Thanks for the replies. sig_a, I have also cleaned and relubbed all of my linkages and the bell crank. I know that the problem is down stream of disconnecting the accelerator linkage from the throttle cross bar because when I do that, the high idle still exists and pulling back on the throttle crossbar does not have any affect.

cheers,
Michael

[304065]

Michael,

I've been following your issues. I've been chasing a 2000 rpm idle for about three years.

Set the advance to TDC (zero degrees advance) at idle. While this might not give you the required 35 at 6K, for diagnosis purposes it will help. The trouble is that the slightest overspeed kicks in the advance and that causes the idle to rise even higher.

The only other problem is that your throttle bodies are pulling too much air with the plates closed. Have you removed the throttle bodies to verify that the wear isn't excessive? It takes about two hours the first time and about 45 minutes when you have done it 10 times like I have.

The ONLY other reason besides advance you could be idling too high is too much air being pulled through the TBs at idle. This is so basic as to be axiomatic, yet it took me a lot of thinking to realize. Either dial back the air corrector screws or adjust the throttle stops for each plate. I made a homemade flow bench out of a shop vac and used my STE synchrometer to set the plates to approximately the same angle before installing them. Now that my idle is high again, I'm going to check the advance and then if that's not it, remove the TBs and zero out the plates.

Good luck! I plan to do this task on Thursday so I should have some Kung Fu to share next weekend.

[mhackney]

Thanks John. So, since I idle at 1600rpm, should I set the advance at TDC at 1600? I'll reset the advance again and then proceed with the throttle body dismantle. I am relatively lucky in that Eurometrix/Matt Blast is about 2 miles from me. If something is up, I can get them R&Rd.

cheers,
Michael

[mhackney]

Forgot to mention, what I actually did to set the timing was to pick off the advance from the advance curve at 1600rpm (which is +5 degrees) and set the timing. If you look at the advance curve I measured (link above) you can see the results. But the idle had not dropped after setting. But now with adjsting the throttle linkage that was out of whack and removing the mouse plugging the thermostat hose, I need to check and reset the timing.

cheers,
Michael

[mhackney]

Also forgot to mention, I did get new gaskets for the throttle bodies so I am all set to give that a shot. Sounds like a good project for the 4th!

cheers,
Michael

[boxster03]

Grady would be the man here to get all of the issues on the table that could be a problem. I would concur that too much air could lead to this problem which could be something as simple as a bad/worn gasket.

[Howard M]

I strongly recommend taking apart the distributor to clean/lube the advance weights and their bearing surfaces, if this hasn't already been done.

Howard

[mhackney]

Thanks Howard. I have not taken the distributor apart. A little hesitant to do so! But maybe I should just go for it. I was thinking of sending it in for a rebuild just to rule that out.

cheers,
Michael

[304065]

No sense tearing down the distributor (an all-day job, and figure three hours to remove the circlip holding the advance mechanism) until you have verified that the advance is sticking. Easy enough to check with a timing light.

[mhackney]

Thanks. Can you take a look at the post I linked to above on my distributor advance curve? Does that look normal? It does to my novice eyes?

thanks,
Michael

[jstobo]

A suggestion. back off your microswitch adjustment screw. Many owners mistakenly adjust the idle with this screw which is for activating the stop solenoid. If it is screwed down too far you will have a high idle. The idle on a MFI engine is adjusted with the throttle bodies air bypass screws on each cylinder. I am surprised to car idles at 1600. This is the range where the RPM Transducer should be cutting off fuel. The idle should be hunting betwwen 1500 RPM and 1300 if the microswitch, RPM transducer and Stop solenoid circuit is working. You probably have some of these electrical problems also. CMA has the procedure to check the stop solenoid circuit. I hope this helps. Good Luck.

[mhackney]

thanks jstobo - something else to look into. I know I haven't touched the microswitch but who knows what the PO did. I have looked at the microswitch but can't recall what I did. Probably best to go back and measure.

Probably the order to look into things at this point is:

1) the microswitch/RPM transducer/Stop solenoid circuit
2) timing
3) throttle bodies
4) dizzy (assuming others agree that the advance curve I linked to in a previous post is not horribly bad).


cheers,
Michael

[304065]

Michael,

Just loosen the lock nut and remove the screw from the throttle arm for testing. Put it somewhere safe. That way it can't hold the throttles open.

The "speed switch" circuit itself doesn't have anything to do with causing a high idle. Remember, the speed switch provides output power when the revolutions are above 1500 rpm and switches off when the revolutions fall below 1300 rpm. The output power flows to the microswitch, which is normally open, but if it's closed, i.e. the throttles are at idle and the screw is actuating the switch, then power flows to the stop solenoid on the pump which moves the rack to cut off the fuel. None of which has anything to do with a high idle: it merely cuts off the fuel when the throttle is closed and yet the engine is still at high revs, such as on overrun, to prevent backfires.

Timing and distributor are the same. If you can, set timing to TDC for testing.







Setting it to zero should keep it from creeping up on you while you determine the airflow. If you raise the revs to do a synchronization test and the timing doesn't return to zero, well, then it's time to tear into the distributor.

[mhackney]

Ok John, I may be completely confused here! Maybe I am completely misunderstanding "timing". Assuming the engine is properly warmed up;

My idle is 1600rpm. So, using the timing light, I set the advance to 4 degrees. If I raise the revs and then let it back down, it will always come down to 1600 with 4 degrees advance. Is this what you are talking about? I am not sure how to set the advance to 0 since that requires an idle of 950 correct?

[304065]

Check the timing at 1600, what is the figure?

Now dial back the timing a few degrees and tighten the clamp gently. Turn off the engine and let it sit for a couple seconds, then restart. Idle still 1600 rpm?

What you're trying to do is determine whether the distributor advance is causing the engine to speed up. So you start by retarding it. Most distributors, when the springs and weights wear, show too MUCH advance, so by dialing back the idle timing, you probably don't bring in the 6K point that much. That's the idea, anyway.

[mhackney]

Thanks John, I can work with that. I know (but will double check) that the timing is 4 degrees at 1600. I can take it from there. So, if I turn back to say 2 degrees, then restart, if I still idle at 1600, it is probably not the dizzy, right? If the idle drops, then the dizzy may be a culprit.

[Jeff Higgins]

Originally posted by mhackney
I have read and tried to follow the Check, Measure, Adjust proceedure.

I really need some guidance on how to proceed before I go crazy!

Cheers,
Michael

Does "tried to follow" mean you were successful in completing the process, or that you tried and were unable to finish? You absolutely must go through the CMA procedure in order, completely, when working with an MFI setup of unknown history. If you are unable to complete it for some reason, find some one who can. It's that important.

That said, CMA was written for relatively new systems. It's amazing what you might find after some one has been "tuning" one for awhile. With that in mind, and recalling you mentioned you were going to remove the throttle bodies, it would pay to check them. Two things are typically going to give you problems.

One, worn throttle shafts that are leaking air. The butterflies should not rattle or show any play in the housings. If the shafts are loose, you will need to rebuild or replace the throttle bodies.

Two, the butterflies do not fully close. Some one has cranked down their individual stop screws, they are dirty, rods are bent, they bind, whatever. With the butterflies "closed", hold them up to a light. If you see any light around the edges, there is a problem. The butterflies should seal the bores completely. The fore and aft edges of the butterflies are angled in opposite directions so that they seal the bore at a hair less than 90 degrees to the bore.

The stop screws on each butterfly should stop the butterfly, not the butterfly itself bearing on the bore. This will wear a groove in the bore that will allow air to leak past. Check for this groove; if it's there in any of the bores, you might need new throttle bodies. The two short rods off the center butterfly should not stop the fore and aft butterflies, either. In other words, their cross shafts should rest on their stop screws with no preload on the two rods.

Once the throttle bodies are verified as good, it's time to put them back on and go through CMA in its entirety. Checking and adjusting things piecemeal, out of order, will only lead to frustration. MFI tuning rewards a methodlogical approach; there are no shortcuts.