MFI idle uncertainty/variation
 

Looking for some more words of advice with my MFI system. I am still having some problems despite many hours of adjustments. Something is not right either in my adjustment technique or the system!

I am experiencing inconsistent behavior of the MFI system which is most noticable as a variation of the idle speed from stoplight to stoplight. I spent some time this evening driving around and observing its behavior. First red light I see around 900 rpm, next red light I see 300-400 rpm less, next stop light it might be even lower or back up to the original 900. Sometimes a couple revs will bring it back up to normal idle if it is low, but not always. This sort of thing drives you nuts when you are trying to make adjustments!

I also get mild and unpredictable hesitation on acceleration but that is hard to quantify!

I tried richening up the idle mixture a bit (2 clicks) because the idle was a bit choppy and this alone didn't seem to fix things. I richened it up more (2 more clicks) and got some popping in a couple stacks, so I adjusted the correlation. This brought the idle speed up around 1000 and I decided to leave it there.

Drove it around town more and it started running a bit better. There is still some variation in the idle but now it is more like 1000-1100. I am also getting an intermittent POP on deceleration. Main mixture is probably a little bit off or there is still a correlation problem?

I have also had problems with the idle mixture, where one day I would find the idle running way too rich, and the next (or even hours later) it would be much too lean. This would be a difference of 4-6 clicks in either direction on the idle mixture screw. I have suspected the thermostat as being the cause of this (prior post) but recently every time I have checked it with the engine above 80 deg C it has been ok. If you note earlier I mentioned I richened up the system 4 clicks to get it to idle better - I will not be surprised if tomorrow I have to lean it back out 4 clicks again. Weird, huh?

Has anyone had this experience before with their MFI system? If the correlation isn't spot on will that contribute to uncertainty in the idle from stop light to stop light or would it be more consistent than that? Last time I checked my control rack moved ok, but does this problem scream of a sticking control rack?

This system can be really frustrating but at the same time it's hard to wipe the grin off my face when I'm screaming along at 5k and everything seems to be working just fine, if just for a moment!

tmctguer - If you could lend a hand in person in figuring this out sometime that would be most excellent... This is basically where I am at right now.

Now, I'm no MFI expert. But before you even think about the injection system, you should make sure your ignition system is in top shape.

A sticking mechanical advance system in your distributor could very well result in the symptoms you describe. If the advance does not always come back to the same point when you slow down, you will get varying idle speeds.

Actual advance at idle has a surprisingly large impact on idling speed. You may want to check advance with a strobe light when idle is at 400 and when it's at 900.

Good luck,

Michel Richard

I totally second Michel's ideas. My distributor advance was sticking and made the car run wacky. When the car was cold, it would idle at about 1100 rpm, then when it would warm up, it would idle at about 1500 rpm (sticking at maybe 15-20 degrees advanced I estimate), then sometimes when I'd run it for an hour or so and it would get good and hot idle would drop to about 900-1000 (I figure it got hot enough for sticky gook on the centrifugal advance to let go). Also check your coil, if it is failing, changes in temperature can affect its output & consistency.

Brad

I have thought about the advance being the cause before. I cleaned out the distributor and lubricated both advance mechanisms, and the problem still persists. A spare distributor would be very helpful in this case but I haven't found one yet.

The advance does change as the idle varies, but I assumed that was just because of the vacuum and centrifugal advances changing as they should. I'll take another look at it.

The coil should be fine, I just replaced it and put in a new ignition system/electronic points. No change to the behavior.

Hmmm wierdness indeed. I'd be glad to help ya out (I have a gunson gastester, which helps with mixture setting) but looks like you're roughly 3K miles away :) Sounds like you've redone most of your ignition - I assume your wires are new or in pretty good shape. Another last ditch thing can be to check/replace your main ground strap from the transmission (I've heard it can cause wacky things to happen). Sorry to harp on the ignition stuff, but I've historically found it to be the cuplrit of many wiernesses.

Also you can get some wierdness if your throttle bodies or linkage is worn/has play. As the engine heats up leakage around the shafts and butterflies can cause variations in mixture. Not sure that's your problem though. Perhaps it could have something to do with the fuel shut off solenoid (signaled by a combination of the rack micro switch and the ever so troublesome speed transducer box) pulling your rack back?

I'm an MFI newbie as well so these are all just suggestions, not bible :)

Brad

Idle Roughness
 

To add to the post above, if you haven't crawled under and checked the trans ground strap, or checked the ground from the CDI box to the engine, it's probably worth a try. I just removed my factory CD box because it's been replaced with an MSD, and when I took out the harness I removed the braided ground strap from the tab on the MSD to the coil bracket and the chain cover. The engine would then barely run: sounded like it was firing on three cylinders, with tons of black smoke and plenty of raw fuel dumping out the pipes. I reinstalled a ground strap (actually a factory trans ground) between a motor mount bolt and the chain cover, and the idle came back to a steady 900 rpm and the smoke and backfiring on acceleration went away. You've probably already checked this but I thought it might be good to add.

As to the speed swich I defer absolutely to the Early S Man. I don't know whether the decel solenoid could affect idle, unless it's not returning properly, but that should be a binary proposition, either on or off.