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

(Greg) #1

So my 88 XJS V12 (48,000 miles) still runs very rough and runs super rich. When cold, it idles so-so, and actually revs halfway decent, but once warmed up to 190F in about 20 minutes, it can barely hold an idle and bogs like crazy when I try to rev it. The vacuum is also super low, anywhere from 10 inHG down to 7 inHg.
I’m at a wall, not sure what to do next. I bought it 3 months ago as a project. I’m hoping some of you may be able to steer me to what to check next.


  • I’ve since renewed the entire fuel system. I verified I am getting 36 psi at the fuel rail with ignition on, and I get 33 psi at idle. I’ve also just had all the fuel injectors cleaned. Injector harness is good.
    Tank and fuel pump were replaced too. FPR is brand new and working.
  • I found the ignition system in rather poor shape. I replaced all HE wires, spark plugs, cap, rotor, the GM part in the ignition amp, the coil, and I overhauled the distributor as instructed in the book. I also replaced a bad pickup, along with a new star wheel. I also checked my timing, got it to 10degrees BTDC at idle with vacuum advance at 800 rpm.
  • I checked compression in October, and all were good, within 180-195 cold.
  • Vacuum at idle and while revving it averages 10 inHg
  • I changed out the CTS, checked CTS, and verified CTS wires to ECU are working. Unplugging CTS or shorting out plug to CTS with paper clip makes no difference.

Car is fouling spark plugs like crazy, and the oil is also getting quite dirty quickly.

thanks for any help, I understand it could be a million things. But I need a list of things to check one at a time, to get this car back to running.


(Steve) #2

To rule out vacuum leak (and I note that you’ve got several other threads where this has been mentioned/discussed), try to bring the engine to operating temperature and remove the B-bank (your driver’s side) airbox cleaner cover. Remove the filter element.
Verify that the engine is running at about 800 rpms as you stated (which btw is not too high idle speed for the v12, should be 700-750).
Next, plug the large hole on the inside of the airbox that feeds air to the AAV (also referred to as extra air valve). Use something sturdy. Tape won’t do it.

When this air supply is cut-off, your engine will barely run, but it WILL run at perhaps 250-300 rpm.
This was a trick an old forum member George Balthrope taught me some years ago when I first joined. If you achieve this, there are NO leaks.

If this is not the case, start chasing leaks with the methods discussed in the other threads of yours and verify that you have the throttle plates adjusted to 0.002 as in “The Book”.

Good luck and Happy New 2019!

(Greg) #3

Thanks, I will do that test. I realize I could be dealing with multiple issues - vacuum leaks, cts issue, ignition issue. I guess one at a time diagnosis.

(Greg) #4

I decided to pull a spark plug, carbon fouled like crazy! I replaced plugs a couple months ago, and have only ran engine a few times.
I know this could easily cause my rough idle. Although it could be running rich and constantly fouling new plugs.
Guess I’ll clean these and see what happens next.

(phillip keeter) #5

Yo Ma,
Sure sounds like an air/fuel mixture problem to me. I did a little search and found a really great article by our own Doug Dwyer. Don’t know if this is old information to you but thought you could take a look.
Good luck,


(Aristides Balanos) #6

You are running very rich indeed.
My first thought is “Coolant Temp Sensor”, measure Resistance vs Temperature and make sure the signal reaches the ECU.

Maybe your O² sensors don’t work and your ECU’s Base Fuel Map is not adjusted correctly, or maybe a bad ECU ?
You are running out of options there…


(Doug Dwyer) #7

Make sure the vacuum hose to the ECU in the trunk is connected and secure. If the ECU sees too little vacuum the fueling will be rich. The vacuum hose originates at the balance tube at the rear of the engine.

Your fuel pressure of 40 psi is a bit high. I think spec is 36 psi. I’m not sure if 40 psi is far enough out of spec to cause a problem, though.


(Gary Crosby 75 XJ6L, 85 XJ-S, 09 XF Supercharged.) #8

Have you checked all the intake manifold bolts. My bolt were loose and causing leaks. May not be the cause of your issue as it would tend to lean out the mixture…but worth taking the 5 minutes to check.



(Greg) #9

thanks for all the info guys. I’ve got a plan of attack.

  1. test CTS (even though it’s new) and CTS wiring back to ECU
  2. check ECU connection (is it corroded?)
  3. replace all spark plugs again (I was running NGK 2238. Should I use NGK 3346? I’ve seen some members recommend those too.
  4. Check vacuum to ECU
  5. Check for vacuum leaks

BTW, I noticed through paperwork that the previous owner back in late 2012 failed smog twice because of running rich. About the time they sold it. It has sat inoperative since then. So I’m obviously dealing with an issue the PO didn’t or couldn’t fix back in 2012.

(Doug Dwyer) #10

Always a good idea to check for intake leaks, yes. But on this type of fuel injection an intake leak wouldn’t cause lean fueling. Incoming air is not measured; the ECU doesn’t know or care where incoming air comes from.


(Greg) #11

Yes, because there isn’t an MAF?

So at idle, does the ECU pretty much adjust fuel/air mixture solely based on the temperature of the coolant and the feedback from the O2 sensors? I hate to throw money at this problem, but will replacing front O2 sensors make a difference?

(Greg) #12

I doubt this has anything to do with my issue, but thought I’d mention:
I’ve removed the air pump system, the A/C, and the cruise control.

(Doug Dwyer) #13


The primary input for fuel metering, at idle or any other mode, is the pressure sensor inside the ECU. All other inputs are trim and/or corrective.

Judging from your spark plugs I’d say you have a pretty severe over-fueling problem. I don’t think O2 sensors would cause it. My impression is that the 02 sensors are trimmers. if they were unplugged entirely the system would default to a preset fueling map… which wouldn’t over fuel the engine.


(phillip keeter) #14

Could his 40 PSI fuel pressure at the rail be causing an “over fueling” condition? If so, how may he regulate that pressure back down to 36?

(Greg) #15

“The primary input for fuel metering, at idle or any other mode, is the pressure sensor inside the ECU.”

You mean vacuum to the ECU?

(Greg) #16

should probably ignore fuel pressure for now, my gauge shortly stopped working afterwards, and I think was faulty. Will get a new one and test again today.
At least my oil pressure sender works. Was getting 25psi at idle, 40psi when revving it.

(Robert King) #17

Actually oxygen sensor signal defaults to middle (.5v) if sensors are unplugged on your system. Good way to test if they are shorted internally to heater is to unplug both. Air leaks in manifold upstream of oxygen sensors can cause rich running. Also check left fuel pressure regulator hose and temp sensor on fuel rail- gasoline in vacuum line is a sure sign regulator is bad. Bypass the temp sensor on rail by connecting the 2 vacuum lines together, if sensor has failed, can cause the fuel pressure to increase by 4-5 psi.

(Greg) #18

Thanks, will check that too.

I just tested CTS and ATS. At 55F CTS read 3.3, ATS read 3.7. checked ECU pins 5,19 and 20,21, got exact same readings. So those check out.

(Robert King) #19

Yes, those sound ok.

(Greg) #20

So I started her up today to test vacuum, and after only a few minutes, she ran out of gas! I put 7 gallons in a few weeks ago, and have only idled/revved it in the garage, probably 15-20 minutes max three times. Could I run that rich to go through gas that easily?!