Annoying problem diagnosis help needed

[Michael]

OK, here is the thing. My car upon a cold start has a surging. This normally would not bother me because I always let it warm up and once it warms up and the O2 sensor is active, the cars runs flawless, but during warm up, the surging gets so that the engine will die unless I'm there to bump it every so often. Even though the problem was there for a while, all last year this was a non issue as it never even bothered to offer me any trouble...turns out the microswitch(I assume from the looks of it, the purpose is to tell the ECU the engine is at idle) had a pin broke off and was disconnected. I made a new blade and re-connected it and now the problem is noticable again. I'm wondering since the car starts and runs perfect cold and warm with the microswitch disconnected, why did I bother hooking it back up? Anyway I would like to fix this unless it involves me going through dozens of vacum tubes looking for a leak or ripping apart the fuel injection system. If all this switch does is save me a little gas on warm up, then I may just unhook it and go back to a problem free idle.

Thanks in advance.

[jawn101]

OK, here is the thing. My car upon a cold start has a surging. This normally would not bother me because I always let it warm up and once it warms up and the O2 sensor is active, the cars runs flawless, but during warm up, the surging gets so that the engine will die unless I'm there to bump it every so often. Even though the problem was there for a while, all last year this was a non issue as it never even bothered to offer me any trouble...turns out the microswitch(I assume from the looks of it, the purpose is to tell the ECU the engine is at idle) had a pin broke off and was disconnected. I made a new blade and re-connected it and now the problem is noticable again. I'm wondering since the car starts and runs perfect cold and warm with the microswitch disconnected, why did I bother hooking it back up? Anyway I would like to fix this unless it involves me going through dozens of vacum tubes looking for a leak or ripping apart the fuel injection system. If all this switch does is save me a little gas on warm up, then I may just unhook it and go back to a problem free idle.

Thanks in advance.

That's interesting that when the switch is disconnected the problem goes away. Have you tried unhooking it? Does the idle immediately settle if you unplug it while the car is running?

[Michael]

That's interesting that when the switch is disconnected the problem goes away. Have you tried unhooking it? Does the idle immediately settle if you unplug it while the car is running?

I have never tried to unplug it during the short time frame of a cold engine, but considering that it starts and idles perfectly when cold with it unplugged, I would assume that it would.

[jawn101]

I have never tried to unplug it during the short time frame of a cold engine, but considering that it starts and idles perfectly when cold with it unplugged, I would assume that it would.

Hey, give it a shot. Won't hurt anything but might be valuable troubleshooting data.

[Bitsyncmaster]

If your taking about the idle switch then disconnecting it would give you vacuum advance at idle. That will probably bump your idle above the capabilities of the closed loop idle system.

[jawn101]

Hmm. So when the idle microswitch is engaged, it disables vacuum advance. But when his switch was broken the idle was smooth. I wonder, Michael - did you try to adjust your A/F mixture during the period when that microswitch was functionally suspect? Maybe you adjusted it in a way that compensated for the extra advance and now it needs to be redone.

[Ron]

Could easily be you simply need to reset your initial timing after the repair.

When the engine warms up, the Thermal Vacuum Control Valve allows vacuum to go to the Solenoid Valve. At Idle, the Idle Speed Micro Switch tells the Solenoid Valve to prevent the vacuum from going on to the distributor advance mechanism. These two conditions, warm and at idle, are required to set the initial timing. So, if your timing was set when the pin was broke/disconnected, vacuum was being allowed to get to the distributor when it should not have been, making it higher than it should be. Not knowing this, the mechanic would unknowingly compensate for this as he set the timing with a light... And when you fixed the pin/connection, the switch would begin to work, the advance would loose the "compensation", and throw the timing off.

(Note this is not the micros only job...but it sounds like you don't want to get into that ;-)

[Michael]

I have never touched it, but I can't say as much for anyone working on it. I have never brought it in to the repair shop for anything like that(like I said, it never bothered me that much) but that's not to say that he didn't try to mess with it. I guess the next step is to borrow a timing light and check it out. I should mention a few things, remanufactured car with stage II engine(hotter cams) and I remember a long time ago reading that in those days James set the mixture to ensure a smooth idle after warm-up, but not so much when cold. The idea was that it's much better to have a surge for the first few minutes, rather than during the whole time the engine is at running temp. This was a long time ago so I'm not sure how much of that I remembered correctly.

The car has always done this since I owned it, which is why I thought this was a normal thing, but the surge was not as prominent as it is now.

[Ron]

FYI- The hotter cams would probably amplify the symptom(s).
I'd check all of the specs (timing, CO, pressures during warm up,etc, in order) and go from there...

[Michael]

FYI- The hotter cams would probably amplify the symptom(s).
I'd check all of the specs and go from there...

I will do that soon and let you know what I find.

[Michael]

Update: I don't know why, but the problem seems to have worked itself out. For the last several times I have took it out, the car started right up and idled perfectly...no surging during warm up. I don't know if this is because it is warmer now than it was a month ago or maybe the computer compensated for the unplugged micro switch and now after being re connected it took several cycles to readjust?....if this fuel computer is that complex. I am basing my assumption on modern day fuel.computers that sometimes need several cycles for an adjustment to take effect..sometimes the adaptation needs to be reset via diagnostic plug, but in any event it was never a real issue to begin with so if it ever does reappear I will take the next step then.

[David T]

All K-Jetted Deloreans will surge on cold start-up. All that fuel dumped in during the starting cycle from the CSV. After a couple of cycles it should settle down. On the Stage II it will be more noticeable because of the cams (higher overlap). That also means they are more sensitive to vacuum leaks. The "computer" has no built in learning or compensation so that can't be it. My guess is the ambient temperature is different now (warmer) so it has affected how quickly the motor warms up. Once the warm-up cycle is over it will run a lot smoother in closed loop.
David Teitelbaum

[Michael]

All K-Jetted Deloreans will surge on cold start-up. All that fuel dumped in during the starting cycle from the CSV. After a couple of cycles it should settle down. On the Stage II it will be more noticeable because of the cams (higher overlap). That also means they are more sensitive to vacuum leaks. The "computer" has no built in learning or compensation so that can't be it. My guess is the ambient temperature is different now (warmer) so it has affected how quickly the motor warms up. Once the warm-up cycle is over it will run a lot smoother in closed loop.
David Teitelbaum

Ahhhh, I was hoping you would see this and offer your take. Yes my car is a Stage II and yes it is much warmer now than it was just a short time ago. I did in fact suspect a vacuum leak as the initial cause of the more than usual surging. I swear I read somewhere that James set these stage II cars up so the surging would be more pronounced upon warm up, but the benefit was that there would be no surging at all during normal operating temp...a good trade off IMO. Anyway, I will keep an eye on it and if it ever becomes prevalent again I will look for a vacuum line leak.