Idle adjustment question, Idles high or slowly stalls Options

[BillC]

I have an idle adjustment question for all you D-Jet gurus:

I can adjust the idle air screw on the throttle body so the car idles at 1600 RPM, and it will happily idle at 1600 all day long. If I turn in the screw just a little bit, like 1/4 turn, the car will slowly drop idle speed down slower and slower until it stalls. I can blip the throttle to keep it running, but it just wants to slow down and stall. This is all when the car is fully warmed up.

How do I get the car to idle steady around 1000-1200 RPM? 1600 is just too high.

Car is a '73 2.0 with factory D-Jet. I installed new Standard Products FJ67 injectors, and the fuel pressure is 29 psi. There are no leaks in the intake side, and the idle adjustment screw is about 2 turns out (so there's plenty of adjustment available in both directions). The MPS hasn't been messed with, and the ECU looks like the correct unit. All new vacuum lines, fuel lines, pump & filter, and fresh gas. New @JeffBowlsby engine and alternator wiring harnesses.

Known issues: it has the wrong CHT sensor (it's the one that's commonly available) and I think the TPS board has worn traces. I finally found the right CHT and ballast resistor, and I also have a 914Rubber replacement TPS board, and I will install both of them when I get a chance. However, I don't think either of those should keep it from idling at a reasonable RPM.

[brant]

I had the same idle down problem a few years back...
fought and fought with it...

once I rebuilt the throttle body.. eliminated the leaks around the worn butterfly... the problem went away.

how does your butterfly plate look in the TB?

[BillC]

So, more frustration this afternoon, but a small step forward.

I moved the vacuum line from the front port to the rear port. And the off-idle transition does seem to be a little better.

However, the car is still idling high. I played with the idle bleed screw and the knob on the ECU, and got it to the point where it would idle fairly reliably at 1300-1400 RPM. I could sometimes get it down to 1100-1200, but then it would often die after a couple of minutes. It seemed to idle best at 2 clicks counter-clockwise from the melt mark. I believe that makes it leaner, correct?

I did hook up a vacuum gauge. When running at 1400-1600 RPM, the car would idle at 8-10.5 inches of vacuum. Any lower, and the vacuum would drop to to 7-9-ish, and then the vacuum would go toward zero as the car stalled. If I revved it up with the throttle, I could watch the vacuum drop to zero and then rise back up as it settled to idle.

I would have expected the vacuum to be higher at lower RPMs. So, not sure what's going on.

I had the same idle down problem a few years back...
fought and fought with it...

once I rebuilt the throttle body.. eliminated the leaks around the worn butterfly... the problem went away.

how does your butterfly plate look in the TB?

Butterfly plate looks okay. I don't see any light around it, nor is it showing any visible wear on the plate or in the bore. However, the throttle does stick just a little bit when cracking it open. Is that a sign of wear? I had assumed that was the plate sealing against the bore.

[Superhawk996]

I did hook up a vacuum gauge. When running at 1400-1600 RPM, the car would idle at 8-10.5 inches of vacuum. Any lower, and the vacuum would drop to to 7-9-ish, and then the vacuum would go toward zero as the car stalled. If I revved it up with the throttle, I could watch the vacuum drop to zero and then rise back up as it settled to idle.

I would have expected the vacuum to be higher at lower RPMs. So, not sure what's going on.

Your expectation is correct. That vacuum is too low.

You need to find cause and address.

Usual suspects:
Valves not adjusted correctly (ie too tight)
Intake leaks
Throttle body leaks / wear as Brant suggested
Cam is not stock and is way to aggressive for D-jet to deal with.

[BillC]

I did hook up a vacuum gauge. When running at 1400-1600 RPM, the car would idle at 8-10.5 inches of vacuum. Any lower, and the vacuum would drop to to 7-9-ish, and then the vacuum would go toward zero as the car stalled. If I revved it up with the throttle, I could watch the vacuum drop to zero and then rise back up as it settled to idle.

I would have expected the vacuum to be higher at lower RPMs. So, not sure what's going on.

Your expectation is correct. That vacuum is too low.

You need to find cause and address.

Usual suspects:
Valves not adjusted correctly (ie too tight)
Intake leaks
Throttle body leaks / wear as Brant suggested
Cam is not stock and is way to aggressive for D-jet to deal with.

Going to run down your usual suspects with my thoughts and observations, to further discuss and diagnose:

Valves not adjusted correctly (ie too tight) Possible, but I adjusted them less than 50 miles ago, so hoping they haven't gone out of adjustment so quickly. However, they were all a little on the tight side at the start of the adjustment.

Intake leaks I'm leaning against this. D-Jet is essentially a primitive speed-density system, and any intake leaks would raise the idle speed. Since I can make the car stall by just closing the idle bleed screw, I think that shows there are too few (or too small) intake leaks to be causing the high idle.

Throttle body leaks / wear as Brant suggested Also leaning against this, for the same reasons as for the intake leaks above. However, I did order a "new" throttle body from ebay, and I'll either install it or send it off for rebuild and then install it, just to make sure.

Cam is not stock and is way to aggressive for D-jet to deal with. At the moment, I'm leaning toward this. I dug through all the receipts, and found the one for the engine rebuild. Unfortunately, it just says "Camshaft" without any specs.

An overly-aggressive cam would explain the low vacuum at lower RPMs. With the vacuum numbers I saw at low RPMs -- 6-9" Hg -- the engine should have been at much higher RPMs, instead of under 1500.

If I get home early enough tomorrow, I'll try calling them and see if they remember what they put in this engine. But it's been 7 years since the rebuild, so it's a long shot.

In the meantime, what's the easiest way to check the cam specs without disassembling the engine. I'm guessing I can pull a valve cover and put a dial indicator on pushrod-end of the valve rockers. Anyone know the specs of the stock cam?