Carpokes

Im not new to wrenching/tuning, and have some experience with Carbs, Motronic, Digijet/Digifant, etc. Through a couple of threads, I have touched on chasing down a high idle when the engine is hot and I cant seem to repair or tune it out, so maybe I need a sanity check?

'87 NA, starts, warms up, and drives excellent. Reaches the middle temp gauge and the idle creeps up to 1100. Now, this car was original and unmolested by prior owners, and it was surging pretty bad when I got it. So I set about inspecting the vac/efi system and slowly replaced the following:

- All vacuum lines replaced with new clamps. Smoke tested before and after.
- Cleaned and descaled grounds.
- New battery/engine grounds
- New speed sensor. Disintegrated.
- New spark plugs. Mismatched, old.
- New spark wires. Dry/split.
- New DME temp sensor.
- New Idle Control Valve.
- New Jboot.
- New AOS o-rings. Dipstick o-ring.
- New manifold gaskets.
- New injector o-rings.
- New O2 sensor.
- Focus 9 MAF and Sport DME... The Bosch AFM had been monkied with, cap pried off, likely adjusted internally, CO screw uncapped... The DME was likely fine.

Now, several areas that do not test poorly and I left alone...

- Throttle body... no smoke leaking... throttle plate not sticking, springs feel strong.
- TPS... multi-meter and audible testing says its working...
- Cap, rotor, coil, Im not seeing/hearing a miss and these look ok.
- Fuel pressure tests well. I did replace most of the rubber fuel lines for safety, didn't see any gunk or issues with fuel delivery during it.

Now to complicate it?

- While replacing the AOS o-rings, my AOS is kaput... well, it "works" but is cracking and leaks air... I actually removed the vac line and vented the AOS to a filter, taking it out of the loop. I dont think this is problematic for an 8v NA, Im not seeing any crankcase pressure issues- but I would like to eventually restore the AOS and lines.
- Same for my fuel evap/charcoal canister, it was donzo... I also took this system out of the equation for now and capped the vac lines.
- Seeing later cars had their venturi deleted. I also deleted mine and routed the booster directly to the manifold.

So here is where Im at... like I said, the car starts, warms up (correct idle speed), and drives excellent, great power... however, once the car is warmed up and hot, it idles at 1100rpm. Confirmed by my dwell/tach tool. I try to adjust the base idle but my throttle bypass screw is nearly closed... In fact if I fully close it, the car maybe comes down to 1050... but has a bad too rich vibration on startup, so I crack the bypass open a 1/4 turn, which makes the car start correctly, but results in the 1100 idle once warm.

My questions:

Have I altered the vacuum dynamics removing consumers? I deleted my power steering, plus these other temporary vacuum mods, could that change how much bypass is needed? Im concerned my bypass is nearly closed, not lowering idle further, which is against other peoples experience of "2 1/2 turns out"...

I know that in 88 Porsche deleted the NA venturi, but anyone with a pre-88, like me, knows if this also causes high idle? Someone on Rennlist suggested this years ago...

Am I just crazy? My engine shows 135-140 PSI compression across all 4 cylinders, intake, head, and exhaust temps all equal and in ranges that make sense... is this just age, plus my changes?

Do I still have a vacuum leak somewhere that isnt showing smoke?

Other ideas/input/leave it alone? lol

Thanks Carpokes! Sorry for the long ass post- appreciate your responses!

spacecad3t wrote: Wed Nov 05, 2025 5:48 am
...

Focus 9 MAF and Sport DME... The Bosch AFM had been monkied with, cap pried off, likely adjusted internally, CO screw uncapped... The DME was likely fine.

Just to clarify, you replaced the AFM with a MAF? Was the exact same problem there before and after?

johnb wrote: Wed Nov 05, 2025 8:40 am

spacecad3t wrote: Wed Nov 05, 2025 5:48 am
...

Focus 9 MAF and Sport DME... The Bosch AFM had been monkied with, cap pried off, likely adjusted internally, CO screw uncapped... The DME was likely fine.

Just to clarify, you replaced the AFM with a MAF? Was the exact same problem there before and after?

Ah good question. in short Yes.

There isnt much more to the long version. I suspected the AFM was adjusted rich, but I couldnt get the plastic cap off, who ever glued it back on was sloppy and alot of glue was used. I didnt want to damage it.

The first start after the conversion was very promising actually and I thought I had solved it, then on subsequent runs the high idle came back.

spacecad3t wrote: Wed Nov 05, 2025 8:57 am

johnb wrote: Wed Nov 05, 2025 8:40 am

spacecad3t wrote: Wed Nov 05, 2025 5:48 am
...

Focus 9 MAF and Sport DME... The Bosch AFM had been monkied with, cap pried off, likely adjusted internally, CO screw uncapped... The DME was likely fine.

Just to clarify, you replaced the AFM with a MAF? Was the exact same problem there before and after?

Ah good question. in short Yes.

There isnt much more to the long version. I suspected the AFM was adjusted rich, but I couldnt get the plastic cap off, who ever glued it back on was sloppy and alot of glue was used. I didnt want to damage it.

The first start after the conversion was very promising actually and I thought I had solved it, then on subsequent runs the high idle came back.

Strange. Does your car have an O2 sensor? If so does unplugging it make any difference?

johnb wrote: Wed Nov 05, 2025 11:13 am

spacecad3t wrote: Wed Nov 05, 2025 8:57 am

johnb wrote: Wed Nov 05, 2025 8:40 am

Just to clarify, you replaced the AFM with a MAF? Was the exact same problem there before and after?

Ah good question. in short Yes.

There isnt much more to the long version. I suspected the AFM was adjusted rich, but I couldnt get the plastic cap off, who ever glued it back on was sloppy and alot of glue was used. I didnt want to damage it.

The first start after the conversion was very promising actually and I thought I had solved it, then on subsequent runs the high idle came back.

Strange. Does your car have an O2 sensor? If so does unplugging it make any difference?

It does. I forgot to include that. Prior to the MAF/DME change, my O2 did test out of spec from a back probing of it. I replaced it as well. That was actually what put an end to the surging, but the idle was still behaving as I have described.

I have not unplugged the O2 since replacing it, or converting to the MAF... I will try that.

spacecad3t wrote: Wed Nov 05, 2025 11:50 am

johnb wrote: Wed Nov 05, 2025 11:13 am

spacecad3t wrote: Wed Nov 05, 2025 8:57 am

Ah good question. in short Yes.

There isnt much more to the long version. I suspected the AFM was adjusted rich, but I couldnt get the plastic cap off, who ever glued it back on was sloppy and alot of glue was used. I didnt want to damage it.

The first start after the conversion was very promising actually and I thought I had solved it, then on subsequent runs the high idle came back.

Strange. Does your car have an O2 sensor? If so does unplugging it make any difference?

It does. I forgot to include that. Prior to the MAF/DME change, my O2 did test out of spec from a back probing of it. I replaced it as well. That was actually what put an end to the surging, but the idle was still behaving as I have described.

I have not unplugged the O2 since replacing it, or converting to the MAF... I will try that.

It's worth a shot, since it's free to check. I suggested it because I'm assuming you're either getting too much fuel or too much air, but it's only happening when warmed up. The temp sensor would have been a more obvious one but you replaced that.

I was going to suggest a stuck ISV too, since that would explain how you could be getting more air without seeing a vaccum leak. But you replaced that too!

Anyway if it was too much air, then I would have thought you could get it to idle perfectly with the idle screw. But it sounds like it goes from idling too high to not running properly at all when you close it.

I will say it's always worth testing things like the temp sensor at the DME harness even when they have been replaced just in case it's the wiring that's the issue.

It's a tricky one.

Appreciate the input. I do suspect my harness has issues... I'll try the unplugging the O2 again.

Have you checked the throttle cable to make sure that it is not too taunt and pulling open very slightly when the engine warms up and expands? Assuming that is not it, I would be inclined to spray starter fluid on all the usual suspects once the engine is warmed up and running fast. Air must be getting in somewhere.

Tom wrote: Wed Nov 05, 2025 6:24 pm Have you checked the throttle cable to make sure that it is not too taunt and pulling open very slightly when the engine warms up and expands? Assuming that is not it, I would be inclined to spray starter fluid on all the usual suspects once the engine is warmed up and running fast. Air must be getting in somewhere.

Thanks Tom! I have run it with the throttle cable slacked up, no change. Before I finally gave in and got the smoke machine I was doing the spray test, nadda, if there is a leak, its somewhere I cant reach easily with spray.

Can you log engine parameters with th le F9 DME?