I have a gremlin that I can't figure out. Have a FI 1.7L that has been rebuilt and all new Vacuum lines, new fuel lines, just updated to a correct part# NOS MPS and it holds 15in/hg forever. Adjusted timing. Checked AAR and it is working. When rebuilding gapped valves to spec.

I have read the threads and main assessment is either vacuum leak or rich/lean mix - could be others but starting with those first. I have clamped all vacuum lines to their attach points so wouldn't expect leaks from these. I read about a smoke (cigar) test, but need more understanding of how that works before I go buy some cigars.


Per https://members.rennlist.com/pbanders/idle.htm, engine stalling when warm says mix is too rich.

Question, my ECU doesn't have an adjustment knob on the exterior, is there an adjustment knob on the inside of the ECU? Want to start with this before I move on to other items..ev

If it is dying when coming to a stop could the decel valve hoses be reversed? I think the side goes to the air box and the large end hose goes to the plenum.

This is probably a dumb suggestion, but did you try increasing the idle RPM?

I think you're on the right track here. Most everything in the FI system is all about getting the correct fuel mixture for the conditions at the right time.


I don't believe the early ECU's had a way to adjust the idle mixture.

Is this happening when cold, warmed up, or at all times? One thing you can do is try to change the mixture and see if it makes things better, worse, or neither. An easy way to do that might be to disconnect the T1 (ambient air temp) sensor. That will enrichen the mixture somewhat, and you can gauge the impact.

Another thing to check is to make sure your TPS is calibrated properly. If it is not, your ECU may not even be "sensing" that it's in an idle condition.

@MDTerp - Great question and one of my contacts mentioned same after posting this. So, my car is a 1971 and per https://members.rennlist.com/pbanders/DJetParts.htm, it didn't come with one. Apparently the ECU is supposed to cut off fuel on overrun. But, I am thinking maybe I need to add a Decel valve and see if that can remedy the issue.

@windforfun - I have adjusted the idle speed at the throttle body but not an idling issue so no impact.

@BeatNavy - it is only happening when coming from high to low RPM quickly. TPS sensing/calibration is something I haven't thought of and is a great idea.

I'm referring the the idle adjustment screw on the throttle body.

Got it. I have adjusted this. Affecting idle only, not high-to-low RPM under load.

This thread may help with decel valve: http://www.914world.com/bbs2/index.php?showtopic=225991

So, to be clear, this occurs, for example, when you are in overrun -- at high RPM, i.e., downshifting or using engine to brake, and then put the clutch in?

^^^ That

You might wind up saving yourself a lot of headaches by swapping for a 72 ECU that had the adjustment knob. 1.7 d-jet parts are still relatively dirt cheap. May have to swap harness and TPS as well (I dont recall when the TPS change was off the top of my head)

The 70-71 cars had issues with this more or less from new which is why they added the knob in 72... you are fighting a 50 year old problem and on top of that your parts are 50 years old now as well which isnt going to make the situation any better.

Plan B is you tune your car specifically for proper idle which then has the side effect of making it run like crap everywhere else, thats the problem when you cant adjust idle mixture independently.

Might have solved the issue..still TBD as I need to put the car through its paces. The engine was running rich - thank you Clay for hooking up your exhaust sensor to measure the AFR.

After some research running rich can be many things but a bad Cylinder Head Temp sensor can cause it. I guess too rich on high to low RFP could kill the engine.

I swapped in a new one and it hasn’t died since. But, its a gremlin so will need some more miles on it to get 100% confident it was the issue.

Thanks for all the input!