What senses the RPM on HT4100 engines?
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HT4100 4.1, 4.5, 4.9 Discussion, What senses the RPM on HT4100 engines? in Cadillac Engine Technical Discussion; Hi everyone-- Does anyone know what sensor reads the RPM for the engine computer (parameter P.1.1) on HT4100 cars? My ...
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    awadecki is offline Cadillac Owners Enthusiast
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    What senses the RPM on HT4100 engines?

    Hi everyone--

    Does anyone know what sensor reads the RPM for the engine computer (parameter P.1.1) on HT4100 cars? My car runs great now, except this idle is still a bit lumpy. It's perfect until 1 mph when the engine slows down. I've reset the TPS/ISC/min air. upwards of 8 times now, 4 times to factory settings, and 4 trying to mess around to get it right. Here's what happens:

    Car runs great all the time except idle right now. As I come to a stop my parameter P.0.1 reads about 2 degrees throttle (engine seems to be turning at the correct speed). Then, at about 1 mph, P.0.1 goes down to 0 degrees throttle and my car begins to idle at about 500 rpms (is this too low?). Then, it starts getting lumpy like it can't find the right idle speed. Sometimes it will even stall. Also, if I jerk the steering wheel hard when the car is in drive, it will stall. I performed the P/S anticipate switch test, and it reads fine, so I'm confused.

    I temporarily "cure" my problem by very slightly pushing on the gas pedal as I come to a stop, just enough to open the throttle switch and the idle is silky smooth. When I release the gas, it starts dancing around.

    I noticed on the freeway when my trans is locked up that my RPM parameter seems to go up and down a bit when it should be reading constant; my hypothesis is that whatever sensor(s) allows the computer to calculate this parameter is messing up my idle, since it is incorrectly perceiving RPM blips.

    I have a completely new distributor in the car including the gear. Car runs awesome otherwise (nice and peppy). Have driven it about 600 miles since putting in the new distributor, plugs, oxygen sensor and wires, and it performs awesome otherwise. I even get upwards of 26mpg on the freeway at 65-70 mph--and that's actual mileage, not what's calculated by the computer!

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    noahsdad's Avatar
    noahsdad is offline Cadillac Owners Connoisseur
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    Re: What senses the RPM on HT4100 engines?

    I've scoured the FSM and can't find the source for engine rpm readings. It's got to be pulse code from the distributor - at least that's the way the old aftermarket analog tachs used to work. I'll keep looking, this is bugging me now.

    With regard to your low idle, does it do it at cold and hot engine temps, or just after it's warmed up?

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: What senses the RPM on HT4100 engines?

    Hi--

    Good question. Actually, it only occurs when hot. I think this is because the idle doesn't slow down as much when I come to a stop.

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    peteski is offline Cadillac Owners Enthusiast
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    Re: What senses the RPM on HT4100 engines?

    Yes, on the HT4100 there is a signal from the distributor to the ECM that tells the ECM the rpms. It might even be the TACH signal. If it is a separate signal then it is just like the TACH signal. One pulse per spark. I'm not near my FSM so I can't give you the specific info.

    If you don't have the FSM, you should get one. It gives detailed explanations all the car's systems. In this example, this functionality woudl be explained under the ECM diagnostics (listed under one of the codes related to possible rpm reading related codes).

    Peteski

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    Re: What senses the RPM on HT4100 engines?

    Quote Originally Posted by awadecki View Post
    Hi--

    Good question. Actually, it only occurs when hot. I think this is because the idle doesn't slow down as much when I come to a stop.

    One simple thing to check that often gets overlooked is to make sure the Thermactor valve is opening. This is the little door located inside the air cleaner snorkel. It should be closed when the engine is cold, open when it's warm. If it gets stuck closed it will starve the throttle body of fresh air.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: What senses the RPM on HT4100 engines?

    noahsdad: Thanks. I checked the Thermac. Seems to be working fine. Closed when cold, open when warm.

    Any other ideas? Again, these symptoms only occur when the car is warm. Also, car idles fine in park when warm, I just have a problem when I'm in a driving gear.

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    Re: What senses the RPM on HT4100 engines?

    I guess the next step I'd take is to compare the o2 sensor output from cold to warm engine: P.0.7 voltage, P.1.3 cross counts, and P.1.4 fuel integrator. You may be getting erratic readings causing the engine to search while it cycles from lean to rich. 02s go bad frequently, because they work in pretty harsh environment. Some techs just replace it as a matter of routine when the customer complains of poor idle.

    Otherwise, a leaky injector perhaps? Do you notice a particularly rich smell as the car warms up?

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: What senses the RPM on HT4100 engines?

    I don't think that an injector is leaking, but I'll recheck. I didn't notice any drips when I had the key on and the air cleaner off.

    I think another cause could be a pesky vacuum leak (hate those), but the car seems to pull really nice, stable vacuum in Park.

    The 02 sensor is toggling between rich/lean and looks to be replaced, but I could try a new one.

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