Why does my D-Jet run too lean? Options

[andreic]

Hello all,

you may remember that I asked several times for help after my 1972 1.7L engine with the original D-Jet injection system developed some problems a few months ago -- falling on its face (badly) upon acceleration, hesitation at constant speed, low power, sputtering, backfiring when cold, etc.

I have narrowed down the problem by now to the fact that it must be running lean. What I did was I disconnected the CHT sensor and installed instead a 5k potentiometer, so I can adjust from the car what the brain thinks the CHT sensor tells it. With careful adjustment of the potentiometer the engine runs like a charm (but I have to keep changing it as the engine warms up). So this rules out many other things (ignition issues, for example), but of course this is not a permanent solution.

The other night I got the engine nice and warm, and measured the resistance of the potentiometer (at the setting where the engine was running well) versus the actual CHT sensor. Big difference: for the engine to run well, the pot was set at 500 Ohms, while the CHT was at 150 Ohms. This surprised me: I had thought the problem was that the CHT-to-ground connection was poor, which would have meant the CHT sensor would have been reading too **high**, not too low.

So my current understanding is that the CHT sensor reads correctly (it reads about 4k cold, and about 150 Ohms hot). But now I am stumped as to what else may be making the engine run lean. And before somebody suggests that it is because of leaky vacuum hoses, I will tell you that I have replaced all of them with quality ones recently. And anyway, with an MPS system, vacuum leaks act as if the throttle were slightly open, but the mixture would have to be still correct.

I have also checked the MPS for vacuum leaks (it holds vacuum well) and the resistances are within spec.

From the looks of the car I don't think any of the component parts were tampered with or replaced (so the FI control box is probably the original one) -- and anyway the car was running well when I got it.

The fuel pressure was checked to be 28 PSI very steady. I have been wondering if it is possible maybe that the fuel injectors could be partially clogged. Could this be the thing to blame?

Another thing I thought about was if maybe the other temperature sensor (on the manifold body) could also be at fault, if it is somehow shorted.

If none of these things are causing it, what else could be making it run lean?

Thanks,
Andrei.

[Wyvern]

I had the same symptoms and chased it for ages . Lots of adjusting , rebuilding and parts. Each thing I did seemed to make it 'better" not right but an improvement.
Found that my injectors weren't all making good constant connection in the harness. I cleaned and bent each female spade connector just a bit.
Eventually I pulled the dizzy again (actually changed to pertronix) again and went deeper into it. Cleaned and lubed the injector trigger points and it now runs GREAT.

[DRPHIL914]

Hello all,

you may remember that I asked several times for help after my 1972 1.7L engine with the original D-Jet injection system developed some problems a few months ago -- falling on its face (badly) upon acceleration, hesitation at constant speed, low power, sputtering, backfiring when cold, etc.

I have narrowed down the problem by now to the fact that it must be running lean. What I did was I disconnected the CHT sensor and installed instead a 5k potentiometer, so I can adjust from the car what the brain thinks the CHT sensor tells it. With careful adjustment of the potentiometer the engine runs like a charm (but I have to keep changing it as the engine warms up). So this rules out many other things (ignition issues, for example), but of course this is not a permanent solution.

The other night I got the engine nice and warm, and measured the resistance of the potentiometer (at the setting where the engine was running well) versus the actual CHT sensor. Big difference: for the engine to run well, the pot was set at 500 Ohms, while the CHT was at 150 Ohms. This surprised me: I had thought the problem was that the CHT-to-ground connection was poor, which would have meant the CHT sensor would have been reading too **high**, not too low.

So my current understanding is that the CHT sensor reads correctly (it reads about 4k cold, and about 150 Ohms hot). But now I am stumped as to what else may be making the engine run lean. And before somebody suggests that it is because of leaky vacuum hoses, I will tell you that I have replaced all of them with quality ones recently. And anyway, with an MPS system, vacuum leaks act as if the throttle were slightly open, but the mixture would have to be still correct.

I have also checked the MPS for vacuum leaks (it holds vacuum well) and the resistances are within spec.

From the looks of the car I don't think any of the component parts were tampered with or replaced (so the FI control box is probably the original one) -- and anyway the car was running well when I got it.

The fuel pressure was checked to be 28 PSI very steady. I have been wondering if it is possible maybe that the fuel injectors could be partially clogged. Could this be the thing to blame?

Another thing I thought about was if maybe the other temperature sensor (on the manifold body) could also be at fault, if it is somehow shorted.

If none of these things are causing it, what else could be making it run lean?

Thanks,
Andrei.

SOOO many reasons , but I can share my experience in getting mine dialed in., but saving details right now, usually it was due to a MPS that was not tuned properly.
if you have no vac leaks and you don't have any issues with the fuel pressure, pump etc., then I would say the MPS could be issue. I had a factory MPS set to specs and rechecked but it was too lean so it hunted almost like I had vac leak. had to adjust the MPS and make it a bit more rich so that was why I put in a A/F meter.
other causes- failing CHT sensor, and a failing MPS can go rich(this is the most common) or lean- I had this happen to 2 of them. so even if your MPS seems to be fine on vac and the copper/berilium diaphragm is fine, and resistance is fine on those checks you did, it still could be set too lean and need to be adjusted. if its never been rebuilt(highly unusual at this point) it could still be too lean. Was the epoxy plug removed from over the adjustor screw? if it does not have the factory rivets then its been rebuilt and then who knows if they set it all back to specs. I had 2 early on in my ownership of the car that had been rebuilt and set to specs supposedly, by FIJ(fuel injection Corp) but they both ran lean, very lean.
Get an AF meter and verify A/F ratio, if lean then adjust the MPS until in spec-
see PBAnders site for proper procedure to do this

Phil