My 92 V12 has a hesitation when accelerating from dead stop after stopping at a stop sign. Not a complete stall just irritating. My vacuum gauge reading when idling is about 13 inches. Low as compared to my old days as a mechanic. What is a normal reading when idling. I removed the exhaust gas recycling hardware some time ago to make things easier to get at. Could this have affected the idle mixture enough to cause low vacuum? Ive looked long and hard for a vacuum leak including plugging off each port to see if there is any improvement. The hose to the ECU is connected and is leak free. I've also tried artificially enriching the mix by adding raw fuel through a vacuum port. If running lean this usually will cause a temporary increase in rpm but in this case it did not. Other than the hesitation It runs smooth at idle and cruising.
I've gone through the throttle linkage setup. I may try checking the pot voltage again at the ECU.
Im about to try messing with the ECU idle pot. Is the pot a single or multiple turn adjustment. Is clockwise richer or leaner?
Other suggestions?
Thanks.

 

ECU idle trim is a single turn pot.

Clockwise is lean.

 

Personally I'd first check for correct TPS operation/voltage.

When the throttle is opened the fuel metering....which is primary controlled by the vacuum sensor in the ECU.... can momentarily lag behind airflow. To mask this lag the TPS sends an instantaneous electrical signal to the ECU.

Perhaps the TPS has a dead spot?

Cheers
DD

 

So not lean and ECU vac is tight. But 13" vac is very low. Must be a vac leak somewhere. How's your timing and vacuum advance?

 

For low vacuum I'd suspect incorrect (retarded) timing, a tired engine, clogged exhaust.

A vacuum leak would typically result in higher than normal idle....unless something has been done to counter-act it.

Cheers
DD

 

A vacuum leak on this type of system will not cause a lean mixture condition. And low vacuum will enrichen the mixture.




Not surprising, as a hesitation on acceleration doesn't suggest any issue with idle or cruising mixture but, instead, transitional mixture

And, just to confirm, the throttle blades are evenly gapped and the bores clean?



I missed this earlier when I mentioned checking the TPS.

Was there a drop-out of voltage when the throttle was opened?

Cheers
DD

 

I didnt' see any dead spot in there TPS but I will check again. Back probing the TPS isn't all that easy. and I hat to open up the wire insulation too much. so I tend to hold the probes with one hand while turning the capstan with the other. Easy to make an error there. Yep, cleaned the bores and gapped the throttles per the procedure. One item That's a bit difficult to adjust per the procedure is the capstan throttle stop gap. But it looks to me that as long as a gap exists it should be ok. I've never checked the timing since it is non-adjustable without some modification of the pickup bracket.
I didn't realize the ECU would enriched the idle mix on low vacuum. Interesting, and explains my the results of my test. If true that would negate the common vacuum leak test by using a propane bottle or carb cleaner.
I've made adjustments on the idle speed to see if it would effect the hesitation. I can't tell that it does.
How does the closed throttle microswitch on the left bank work with the ECU?

And thanks for all the suggestions. I won't have time to do much for a couple weeks. It's getting new paint!!

 

Awkward, yes.

last time I was fussing with the TPS I soldered a couple test leads into the wires to make future testing, if needed, easier.


Ah yes. Marelli. I momentarily forgot!


You may have touched on something!

The idle switch, normally closed at idle, works on the ignition ECU to switch timing to idle mode, for lack of a better term. But, the ignition ECU also has a barometric pressure sensor. The baro sensor reading, which reflects engine load, is ignored with the switch closed.

Worth checking to see if the switch is stuck closed, perhaps?

Cheers
DD

 

An old trick here is use sewing pins and insert them through the wire and connect the meter with alligator clips.

 

Are we all talking about the same switch?

The tech guide (attached, page 9) says the idle switch is closed at idle

Cheers
DD

 

Good to see someone is on the ball. I must have had too little red wine when I looked at the diagram as the switch goes to pin 5 of the ECU.