D-jet idle and lean issues. Options

[Literati914]

1.7 w/ original D-jet.

I have the idle screw on the TB cranked all the way down/closed (which doesn't seem right tbh) and the car's idleing around 900rpm - which is within the recommended 850+/- , and the idle does not really "surge" too much but is not necessarily rock steady either. I'm also getting a steady 16.5 A/F on my gauge during idle and warm driving. So too lean. BTW, the CHT gauge has never gotten over 300 so it's not trying to over heat.

I'll be spraying the vacuum connections to see if I can find leaks - but other than that, what would be the most obvious culprit(s)?

My theory is that the idle screw needing to be closed (for a reasonable idle) is just combating say, a bad AAR or something. Sound logical? What tests can I do to the AAR or other items to look at first?

[ctc911ctc]

I vote:

Vacuum leak throttle body and Plenum

Good luck!

1.7 w/ original D-jet.

I have the idle screw on the TB cranked all the way down/closed (which doesn't seem right tbh) and the car's idleing around 900rpm - which is within the recommended 850+/- , and the idle does not really "surge" too much but is not necessarily rock steady either. I'm also getting a steady 16.5 A/F on my gauge during idle and warm driving. So too lean. BTW, the CHT gauge has never gotten over 300 so it's not trying to over heat.

I'll be spraying the vacuum connections to see if I can find leaks - but other than that, what would be the most obvious culprit(s)?

My theory is that the idle screw needing to be closed (for a reasonable idle) is just combating say, a bad AAR or something. Sound logical? What tests can I do to the AAR or other items to look at first?

[rjames]

You may have a vacuum leak, but it sounds like you’ve compensated for that via the idle adjustment screw. That makes me think the vacuum leak is not the cause of your lean mixture at idle.

Did this condition just suddenly happen?

[emerygt350]

Mixture is unaffected by leaks. Idle rpm is. I agree that you are probably looking at a leak in the TB or plenum. Can-o-carb cleaner is your friend. Spray it around the TB, runners, etc, if the idle drops you found it. My car leaks around the idle screw but only there, so I just leave it all the way in and call it even.

As far as your idle mixture.. Do you have the idle mixture pot on top of the ECU, I don't think you do with a 72. And to reiterate above, did this just start all of a sudden? And what is the mixture like while driving? Does it never go over 300? Seems a little cool, like your engine is not performing as it should.

[Ishley]

My ‘72 has the idle mixture knob on the ECU.

[emerygt350]

My ‘72 has the idle mixture knob on the ECU.

Well, you could richen it with that. I suspect as the engine ages it probably has different requirements from when it left the factory. Have you tried some PEA fuel system cleaner? Like the 10$ gumout stuff? Maybe your injectors are getting gummy. A lean mix is gonna play havoc with idle, could be as simple as a bad tank of gas....

[Literati914]

My ‘72 has the idle mixture knob on the ECU.

Well, you could richen it with that. I suspect as the engine ages it probably has different requirements from when it left the factory. Have you tried some PEA fuel system cleaner? Like the 10$ gumout stuff? Maybe your injectors are getting gummy. A lean mix is gonna play havoc with idle, could be as simple as a bad tank of gas....

Yes like @Ishley confirmed, my '72 also has the knob - it does not "seem" to do much of anything to the idle though ... actually I'm not sensing any clicks from it like you are supposed to (maybe the knob's receiver hole is worn out). I will go back and examine that for sure though. I moved it CW while engine was running and watching the gauge and there was no change at all. But besides, the gauge NEVER shows anything but 16.5 even when out hustling it around the neighborhood - and the knob is only for idle (so I admittedly put little importance on it as being the root cause of the lean gauge numbers).

I have not used any injector cleaner - but gummy or clog(ging) injectors are possible as I've had to clean the fuel filter twice now from a buzzing fuel pump. My tank clean before install obviously left rust-dust particles behind. The pump is quiet after the filter clean and the car runs with plenty of pep - pulls surprisingly well tbh. I've purposely left the fuel level low to help evacuate remaining rust micro particles. I wish the bottom of the tank was ever-so-slightly funneled to the output fitting.

Oh, I did raise by fuel pressure from 27-28 to 30psi, which helped smooth idle but did not richen my mixture, according to the gauge.

[Literati914]

You guys asked if this condition just started - well no, not really. What I mean is that the car has only been driven around my neighborhood about 6-7 times since the car has been restored, etc. The first couple times, A/F didn't seem dangerous at 16.5 so I was concentrating on all the other stuff. But with those numbers I haven't pushed it real hard - probably why I'm only around 300 cht.

[Ishley]

My 72 ECU knob does nothing. I have an Air fuel gauge and I see nothing from turning at idle. My guess is the knob is toast. I’d replace it but overall I’m happy with how it runs and the mix that I see on the AFR. I did spend a lot of time tweaking my Manifold
Pressure sensor to get it dialed in. This was necessary because I rebuilt to a 2056 with a different cam and heads and 2.0 injectors.

[rjames]

You guys asked if this condition just started - well no, not really. What I mean is that the car has only been driven around my neighborhood about 6-7 times since the car has been restored, etc. The first couple times, A/F didn't seem dangerous at 16.5 so I was concentrating on all the other stuff. But with those numbers I haven't pushed it real hard - probably why I'm only around 300 cht.

Did restoring it include a motor rebuild or a new mps?

[JeffBowlsby]

A reminder that the ECU idle mixture knob is only active when the TPS is correctly calibrated. Two of the circuits in the TPS are a switch that tells the ECU to control mixture at idle, and you should see a difference in the lean/rich condition at idle under that condition. Off idle, the TPS turns off the ECU-controlled idle mixture function, and directs the other sensors combined (MPS, TS1, TS2) to control mixture as determined by the monitored engine conditions.

If there is no reaction from the ECU knob at idle, then the TPS is likely not calibrated correctly.

[Literati914]

Did restoring it include a motor rebuild or a new mps?

The car was in need of restoration but the engine was out of the car and supposedly freshly rebuilt .. but no receipts. It was certainly clean and some items obviously new. I believe the MPS is original .. apparently untouched also. It’s the correct model for my ECU, which is correct for the ‘72 as well.

[emerygt350]

What Jeff said. That is a good sign that your TPS is not adjusted correctly. Many afr gauges default to something like 16.5 when the meter is out of range or it doesn't like the o2 sensor. What brand? Does it move at all ever? If your car starts cold I can guarantee the mix isaround 12:1 at start up. If the gauge is telling you 16.5 it isn't working.

[Literati914]

A reminder that the ECU idle mixture knob is only active when the TPS is correctly calibrated. Two of the circuits in the TPS are a switch that tells the ECU to control mixture at idle, and you should see a difference in the lean/rich condition at idle under that condition. Off idle, the TPS turns off the ECU-controlled idle mixture function, and directs the other sensors combined (MPS, TS1, TS2) to control mixture as determined by the monitored engine conditions.

If there is no reaction from the ECU knob at idle, then the TPS is likely not calibrated correctly.

Hey Jeff - it’s interesting that you brought up the subjects of the knob “only changing idle mixture”… why? Because I just spent some time with my ecu and got the knob to the center spot (actually maybe 1 click rich). To do this I had to move it CCW from where I had it on my last run. It did not change the 16.5 at the gauge while idleing (or driving). But this time while test driving the car it tended to want to back-fire and pop!

Two things with that: the knob shouldn’t effect driving mix, and I thought back-firing and popping were indicative of being over rich (I moved ccw leaner) ?

I messed with attempting to configure my TPS before installing my replacement TB last week. Here’s a pic of how mine lined up:

[Literati914]

A reminder that the ECU idle mixture knob is only active when the TPS is correctly calibrated. Two of the circuits in the TPS are a switch that tells the ECU to control mixture at idle, and you should see a difference in the lean/rich condition at idle under that condition. Off idle, the TPS turns off the ECU-controlled idle mixture function, and directs the other sensors combined (MPS, TS1, TS2) to control mixture as determined by the monitored engine conditions.

If there is no reaction from the ECU knob at idle, then the TPS is likely not calibrated correctly.

Hey Jeff - it’s interesting that you brought up the subjects of the knob “only changing idle mixture”… why? Because I just spent some time with my ecu and got the knob to the center spot (actually maybe 1 click rich). To do this I had to move it CCW from where I had it on my last run. It did not change the 16.5 at the gauge while idleing (or driving). But this time while test driving the car it tended to want to back-fire and pop!

Two things with that: the knob shouldn’t effect driving mix, and I thought back-firing and popping were indicative of being over rich (I moved ccw leaner) ?

I messed with attempting to configure my TPS before installing my replacement TB last week. Here’s a pic of how mine lined up:

[brant]

16.5 at all conditions seems wrong

Idle mixture doesn’t matter
At load does

My gauges. A few of them.
Move wildly with application or removal of throttle

Stuck at 16 is not right

Have you calibrated recently?

I’d put a new sensor on it to see if there is a change

[emerygt350]

Did you set the TPS according to the instructions? You need a multimeter for that. Super easy but fiddly. Off hand, having it maxed out like the picture seems to be a doubtful position.

Does your mps have the rivets still in place?

[JeffBowlsby]

That TPS board is worn out, should be replaced.

[Ishley]

You might want to try a new O2 sensor. I’ve have had several go bad when I was running too rich. If you decide to change it… I’ve had the best luck with a Bosch.

[emerygt350]

I have been using disposable Amazon o2 sensors. 29 dollars. As long as the car isn't running really badly they last years.

This is the last one I bought.
https://a.co/d/aSQhnMZ

You should at least use an eraser on that tps