Stock 72 1.7L D-Jet that is currently running well overall right now. But I have an odd issue where sometimes the idle will surge wildly when the engine is cold. It only occurs very inconsistently, but when it does, it happens the same way:

I start the car and it idles and runs fine as I'm heading to the end of my residential street (call it less than a 1/3 mile). While I'm going slowly in 2nd gear, the car will almost like clockwork buck once or twice. No big deal. When I get to the end of my street and stop at the intersection, the idle may suddenly start surging up to around 1800 RPM and immediately crash and surge up again. The car's probably only been running for about a minute or two at this point. Once I accelerate there are no symptoms or drivability issues, and after the car's been driven for a 4 or 5 minutes it doesn't seem to happen. It also only occurs MAYBE every other time I drive the car if even that. It only started about 3 weeks ago. I did replace the TPS Board roughly around that time.

I'm not 100% sure I calibrated the TPS correctly after installing the board. I followed the instructions on Pelican (if there is ANY ambiguity in any instruction I'm sure I'll find the way to screw it up). On step #5 of the instruction it says to "move switch CCW exactly one more hash mark." I assumed this means those little marks on both sides where the screws hold the switch in place. I don't want to focus totally on this as a likely culprit, but there does seem to be some correlation (time-wise) to this occurring and when I swapped out the TPS board.

Also, I do have a CHT spacer that helps with warm-up.

Any thoughts? I don't see a similar symptom mentioned on Anders' site.

In other exciting news, it appears I have been promoted on 914World - now that I have 50 posts it looks like I get TWO squares under my name

You've probably double checked there are no vacuum leaks. Since you put in a new TPS, it does seem suspect. Can you put the old one back in? Was the CHT spacer on before you made the TPS change? I've noticed on my car if it is running lean, I would get wierd surges and bucking the would change if with the engine temp. You don't mention your ignition. I found with points (I now have electronic), as they wore, it would buck when cold.

I know vacuum leaks are definitely a potential issue. I am certainly not aware of any, and the hose are in good shape, although I have seen similar behavior when I had a line disconnected at one point. But this only happens when the engine is cold.

Putting the old TPS board in is sort of possible, but it would require drilling out the rivets and all that. Anyway, the old TPS board had a bad idle trace. Didn't seem to cause any issues, but it wasn't in great shape. I'd hate to resort to that.

The CHT spacer was put in about a month before I changed out the TPS board. I think the ignition is good, but just like the vacuum situation its always possible something happened to it to cause the issue (but again, only when cold). The ignition is all stock.

One thing on my to-do list in general: buy a vacuum tester so I can test the MPS. I've never done that.

Get the Habor Freight vaccum gauge.

Test your MPS before you do anything else.

I had this on my 2.0. My AF guage showed I was going extremely lean when the surging occurred. Used the knob on the ECU to richer it up at idle and all was good.

Might nt be your issue but thought I would pass it on.

You are right. I will buy the MightyVac from Harbor Freight right friggin' now.


Thanks Mike. So I've always suspected I might be running lean. Several months ago I was getting the slight exhaust "popping" on decel especially when cold. After installing the CHT spacer and making sure ignition was right it seemed to be better. I also cranked up the fuel pressure a bit (based on my crappy fuel pressure gauge I had it at like 32 - 34 psi). Come to think of it, the OTHER thing I may have done around the time I replaced the TPS board was back that pressure off slightly. Didn't use the gauge ('cause it leaks) but just turned the adjustment back a little from where I had cranked it up. My ECU is the early kind that doesn't have the knob. From what I recall, without that knob the only way to adjust mixture is fuel pressure and/or figuring out how to adjust MPS, right?

Thanks.

Not sure if it is the same on the 1.7 but on the 2.0 you can pull the wires on the air temp sensor (one that is mounted in the intake) which should richer it up guide a bit. If the surge goes away you know it is a lean condition. If it is still there then MPS check is next.

Just disconnect the air temp sensor? Cool, I can try that.

Glad you started this thread, man. I just tried it, and disconnecting the intake air temp sensor fixed my bucking issue completely after cleaning my dizzy made no difference (although it was still a good thing to do!).
I'll need to measure the sensor with an ohm meter to see if it's failed. I think I was just reading that if it fails and shows as a short between the two terminals on the sensor, it can lean out the mixture.
I still haven't had a chance to check my fuel pressure though, so that could also still be the issue with my car.

Sweet, now that you've both mentioned mixture and air temp sensor I do see a reference to that on Anders' site and will check that out, as it makes sense. Kind of cool to "follow the clues" on something like this. Not that I've solved anything yet, but it would explain a few things. Thanks.

One of the best things you can do is invest in a AF gauge. I resisted spending the $150 for a decent one but it was the best thing I have done.

Even if you don't run the AF all the time it is great to have if you are in a car and FI for the long run.

Thanks, Mike. I'll look into AF sensors/gauges. Something else to learn and play with!

I did pick up my vacuum tester. Harbor Freight has a retail store in our town now

Cool. I would like to claim that was something I just knew but it was passed on to me by a very wise man the Cap'n. If he gives you information on here you can be pretty sure its correct.

Thanks for your help, Mike. In the short time I've been here the Cap'n has already helped me a couple of times directly and more times than he will ever know indirectly (like this time).

As an update: disconnected the temperature sensor. I checked the resistance between terminals on the sensor itself, and it seemed normal (about 320 ohms at 61 degrees). I did notice the air temp sensor part of wiring harness has a burn in it from leaning up against the hot engine and the insulation was burned away, probably shorting it against the engine. I checked the resistance between the connector terminals on the wiring harness to see if it had shorted between the two leads. I got a reading of about 2.1 K ohms. So I guess that's ok, but not sure. Most likely if there were issues it was because one of the leads was grounded against the engine, maybe?

Anyway, I left it disconnected and took it for a spin this morning. Didn't have any issues, so I'm going to keep running this way, see what happens, and adjust the fuel pressure as needed.

Also, I finally did the vacuum test on the MPS I've been meaning to do. From 15 hg it leaked to 10 in about 40 seconds. I think that means it is getting old and tired, and may fail soon, but that it can still sort of do the job it was meant for. Kind of like me.

Thanks for all the help!