My Jag has an irregular idle at times. Sometimes fairly dang smooth then shakes but doesn’t die. As I listened to the exhaust note yesterday, it was what is described as “heavy running.”. And then there would be a “whomp……whomp, whomp,…….whomp” sound, which both are signs of too rich mixture, right? Plus my eyes could water and my clothes stink.
I turned the load screw (most outer) counterclockwise of the manifold pressure sensor to lean out the idle just a little and it was getting better, smoothing out with very little “whomp” sound. [EDIT: I’ve since read that ccw turn richens and cw turn leans?]
But ccw turning it out dropped the idle rpms by about 100 too. Strange to me, since I would think the engine rpm would be highest when mixture is best (engine happy). Anyway, I opened up the air bypass to increase the idle rpm.
At a fixed 2000 rpm, where I had the throttle mechanically held open so I could hear it the cruising exhaust note, the engine wasn’t smooth. I turned the load screw clockwise to richen back up the system and it smoothed out and rpms increased. I removed the throttle stop I was using and idle was way too high.
I lowered rpms back down by turning back in the AAV screw and now the engine has no power. Turn the throttle and the engine to me is suffering from not enough fuel. It tries to climb in rpms but does not rev freely. It slowly climbs. If I hold it too open too fast, it will backfire.
Where did my fuel go?
I’ve turned the load screw in small (1/8) turn increments trying to find a place where the engine will rev and I can’t find it. All the way clockwise, and then counter clockwise (but not too much). engine doesn’t freely rev anymore.
I did the static test of the MPS - holds 15 inHg for more than 10 seconds.
The primary windings measure 99 ohms, supposed to be around 85-95, but that may be close enough? The secondary windings measure 355 ohms which is in specification.
I’ve done a lot of reading on this forum over the last couple of days and out there on Porsche and Mercedes sites. A couple times I have read if you back off the load screw too far, you can cause an internal problem with one of the diaphragm cells. thoughts?
Is there a chart for our ECU where I can measure the ohms with various vacuum pulled on the MPS to see if it is behaving properly? Is this the right approach?
More background data regarding this car;
This car gets driven once or twice a week, but In the two days prior to fiddling, I had put more than 100 miles on it. Running superb, just the erratic idle at stop lights. at WOT, the car accelerated very impressively.
In last year, new Dizzy cap and rotor, wires, plugs. Years ago I converted the ignition pickup to the Lumenition light beam system - amp mounted on radiator crossover piece.
New Fuel pump.
Fuel pressure regulators are set at 29.5-30 psi when fully hot. I have a permanent fuel pressure gauge mounted in the engine.
Throttle linkages are properly adjust opening the butterflies equally.
I used my stethoscope to listen to each injector at idle, and at higher rpm (1500-2000) and each sounds the same with the clicks quickening at higher rpm.
A couple years ago, I traced every wire back to the ECU to ensure I had no resistance and good continuity.
It has been a few years, but I did check the throttle switch to ensure at idle, it is sending the idle signal, and at WOT - it is sending that signal too. (will check again tomorrow, as I’m searching for anything now)
(I also switched fuel tanks just to ensure I wasn’t having some kind of fuel flow problem, no difference)
Can the MPS actually really restrict the fuel flow that much (thinking something else is giving me trouble even though I had only fiddled with the MPS and idle air screw)?
Anyone in the Dallas Texas area have a D-Jetronic V12 running well that I can borrow your MPS to swap to investigate?
Thank you for you ideas and thoughts!