84 V12. Has developed a loss of power and/or vacuum from 1500-2000 rpm under load, seemingly only when the air conditioning is on! You're doing around 30mph and step on the gas, but the pedal input doesn't do much and the engine seems to lag. Concurrent to this, the airflow sometimes diverts from the front facia vent to defrost. No misfiring, no evidence of trouble at the tach. Putting your foot more fully into it, it takes off with a roar as usual. What the heck?

Thanks!

Lou

 

The loss of vacuum is normal on opening the throttle. However, your climate control system should "feel" it because it uses a rservoir and check valve to prevent it from being disturbed by the normal fluctuations of engine vacuum. I suspect, in your case, one or the other is leaking.

As for the running problem is suspect a dead spot on the throttle position sensor. If you have a volt meter I can tell you how to check it.

Cheers
DD

 

Thanks Doug. I don't think it's the TPS because the engine symptom goes away when you switch off the a/c. Plus, it's a brand new TPS that has been working great for about two months. Any other ideas on how the air conditioning system can cause the "flat spot"?

What if the alternator has lost a diode, and voltage to the ECU is going low when A/c is on? New battery, but my tech says the alternator has lost a diode (although I have noticed nothing different in the behavior of the voltmeter and warning lights).

 

Of course I meant to say "....shouldn't feel it......"

 

Ok, that pretty leaves out the TPS. I'll have to do some mulling on this one. Did this problem show up out of the clear blue or was it coincidental with something else?




The fuel injection system does like full voltage but my gut feelng doesn't take me down that path in this case.

Still, it couldn't hurt to fully charge the battery with a charger and see if the trouble persists, and clean the main cables and grounds just on GP.

Cheers
DD

 

It's definitely with the a/c on, only. Switch off the climate control and you're back in full business.

I note the instant gas mileage drops radically during the symptom, as you'd kind of expect.

The only thing concurrent to this fault was the replacement of the a/c compressor, to fix the system.

One last possible clue, I can hear a very slight hiss from the evaporator?? When coming off idle.

Somehow, the a/c is making the engine behave as does a faulty tps would....

 

Well the mysterious symptom has gone away, at least for the moment, concurrent to work on the suspension.

I'm thinking this is an electrical issue--the A/C uses lots of current, and the charging system has a known issue (a bad diode in the alternator). When the battery goes low enough, it "starves" the ECU because of the drain created by the A/C draw.

Still, before replacing the alternator I would like a second opinion.

 

Voltage drop is worth checking but don't buy any parts until you've checked/cleaned/tightened all relevent connections and grounds.

Cheers
DD