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> Why does my D-Jet run too lean?
andreic
post Dec 10 2018, 04:58 PM
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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.
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