Car still idles rough/runs rich after 2 months work - I'm at a wall

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #21

You’re thinking of carburetors. In a closed-loop EFI system, a manifold air leak would cause excess air into particular cylinders. That excess air would pass unused into the exhaust system where it would be picked up by the oxygen sensors. The EFI would see excess oxygen in the exhaust, decide the engine is running too lean, and richen it up.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #22

Agreed, they should not. I will point out that at least one owner suffering similar rich-running issues eventually traced the problem to an obstructed fuel return line to the tank. In his case, the problem was obvious: Someone had jacked up the car using a floor jack under the left side rocker panel and had crushed the fuel return line.



Jump the CTS connector, and the Fuel temp switch connector if you have that. They are wired together. This eliminates either as a source of enrichment. Might as well jump the ATS out as well for good measure.

Confirm your WOT switch is not stuck closed. Pressing it manually should cause audible change to idle due to enrichment. May stall your already rich car.

40 psi on your fuel sounds like the right pressure for the OEM regulator when NOT under vacuum. Ensure that pulling the vac hose off the Outlet side of the fuel rail raises the pressure. This should also make an audible change in the idle as it enriches the mix.

Check the position of the trim pot in the ECU. Is it at one extreme? That would indicate PO fiddling. If so, record the position (count clicks) and then try a centered setting.

As others suggest, confirm your vacuum at the ECU.

You shouldn’t need an adjustable FPR, but if you do wind up replacing yours it would offer another lever to pull. Should be fine with the default pressure if all else is right.

I would pull the whole fuel rail loose and raise it up, turn on the ignition and confirm no leaking injectors. Open the throttle quickly and confirm all fire.

I’m thinking your clue in the original post about the light not firing when you had a miss indicates an ignition issue. This may not be connected to your rich condition. Did this occur on multiple cylinders or just 1A while timing it? Coil, amp, cap, rotor, shorting HD leads are suspects.

(Greg) #24

Will reread all posts and start checking those, thanks! Here are my next tests:

I am using a quality fuel pressure tester now. Am getting 36psi with ignition on, 33psi at idle, and about 36psi if I rev it to 2000.

I checked vacuum hose from engine to ECU. 12psi at engine, only 7psi at ECU. Connected direct hose, no change.

Disconnected o2 sensors, no change.

Removed air intake and covered hole as suggested, engine rpm went way down, but didn’t stall.

With that #1 cylinder wire not firing the timing light, I may start diagnosing that. May need spark tester.

Also, exhaust, although not black at idle, hurts my eyes. :slight_smile:

And I put in 10 gallons of gas. We’ll see how long it lasts.


Kirbert, Would you, similarly, expect a mis-firing cylinder(s) to also cause enrichment because of the air passing through them unburnt?

I assume so, but in neither case when in Park. It would be open loop. No O2 feedback.

(Greg) #26

So the engine idling at 12 vacuum, and the ECU only seeing 7. Could this nake a big difference in air/fuel mixture?

I will look for the tuning pot in the ecu next, is it easy to get to without removing the whole ecu?
And do these ecus need the battery disconnected to ‘reset’ fuel mixture?


12 vac is low. Likely due to barely running. 15 is ok 18 good.

Any discrepancy between manifold vacuum and the ECU end of the hose is bad. You’ll be receiving fuel for a significantly more open throttle than you actually have. Fix this first. Make sure the sensor itself holds vacuum. Then I’d run a line external to the car to the rear of the manifold taps for testing, but I have a spool of vac line.

(Greg) #28

I’ve put a long vacuum line in to test.
But if the ECU thinks 7, and the actual vacuum is 12, wouldn’t that make it run lean?

(Greg) #29

Was checking wires from ignition amplifier to distributor, and noticed this.
The red wire from the distributor cannot mate up with the red wire to the amp based on the plug connection.
My original pickup coil has the connection so the two red wires will connect.
Bought the pickup as SNG part. Did they get their wires crossed, or do I have mismatched amp/pickup?


Low vacuum tells the ecu the throttles are open a lot. ECU increases injector firing duration. No vacuum means throttles are wide open. Injectors open full duration. If you don’t really have the throttles open, but have a leak at the ECU connection, then way too much fuel is delivered for the actual air.

I don’t know for sure about that pickup reversal. I don’t think it matters which way the pickup is wired. It’s a voltage pulse. Can’t say for sure except to say that it apparentlyworks in this reversed connection.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #31

Assuming the ignition system was OK and timed correctly before, even if you did not have the engine running, if the red wire is reversed this happens:
The ignition now triggers on the falling edge of the dizzy pulse instead of the leading edge ( or maybe vice versa ). Whatever, this will throw the timing out by something like 30deg. You can rotate the dizzy by the same amount get the timing back to square one. Actually the dizzy runs at half speed, turn it 15deg.

(Steve) #32

Assuming you’re reporting your measurements in “in Hg”, this level of vacuum is unacceptable.
Have you done compression test?

I am getting 18-20 in Hg vacuum at idle speed (apologies for the missing pictures, forum admins should fix this in 2019)

Do not pay too close attention to my fuel pressure numbers. My beast has the 6.0L and runs the 3 bar FPR. But, when you open the throttle, vacuum should decrease and fuel pressure should increase, which is not quite what you’re reporting.

Check again, please.

(Greg) #33

Yes, my vacuum idle is at 12 in hg, but the car idles very rough at about 8-900rpm. So that could explain the low number. I did compression test two months ago, all 180-195 cold.

I’m currently chasing ignition again, since my timing light was ‘missing’ too. I am still not getting a nice blue spark from the coil, it’s not too small, but is more orange than blue. That translates to a small orange spark to the wire, and sure enough it was intermittent.
Having replaced cap, rotor, wires and coil, I’m wondering about the HEI, could that contribute to poor spark? The coil was getting a nice 13+ V at positive while running.
After I switched off car, I checked resistance between negative coil terminal and ground, and got 4.3 Kohms. Should there be any? How is the coil grounded? This HEI has me confused with all the wires going into it.

(Steve) #34

I am not sure how misfire can translate to low manifold vacuum with closed throttle plates, but the fact that your measurement shows 12 in Hg in the intakes and only 7 at the ECU is suspect.

You also stated in an earlier past that “I am using a quality fuel pressure tester now. Am getting 36psi with ignition on, 33psi at idle, and about 36psi if I rev it to 2000.”

That’s not right. I got grilled on this awhile ago. 1 bar = 14.7 psig = 29.9 in Hg. If you say that your idle vacuum is 12 in Hg, that;'s roughly 1/2 bar or 7 psig. Fuel pressure therefore should drop to below 30 psi if it was 36 psi before the engine was started…

Double check the integrity of the line (first it is rubber from the A-bank intake and then solid metal tube and then again rubber) to the ECU.

(Greg) #35

For now, i’m using a solid silicone line from the engine to the ECU along the roof of the car. Once I solve the issue, I’ll fix the line under the car.
The fuel pressure tester seems good and accurate. Does the ECU control fuel pressure based on vacuum reading?

(Foggyoo) #36

Are you sure your ignition timing is correct?

(Greg) #37

Yes, I set it to 18 btc at 3000 rpm. No diff.

(Foggyoo) #38

Shoudnt it be more like 30-35* at 3k revs?

(Greg) #39

Mine is u.s. 1988 V12, sticker said 18 @ 3000 rpm.

(Doug Dwyer) #40

The ECU doesn’t control fuel pressure. That’s the job of the pressure regulators.

I’d be tempted to remove and plug the vacuum line to the ECU and then use a hand held vacuum pump to apply (at least) 12" vacuum to the ECU, to match the 12" at the engine…and see if the running improves.