XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

Pulsing Idle NOT fixed by new O2 sensors

Old 06-06-2019, 09:03 PM
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Default Pulsing Idle NOT fixed by new O2 sensors

So trying to resolve an approximate 50 rpm pulsing at and just above idle. The Gurus here advised me that they solved similar conditions with new O2 sensors.
So I purchased two new OEM style Bosch sensors from Rock Auto at a very good price $57 each. Bottom Line...after the ordeal below with the new sensors installed,,,the car STILL PULSES.......damn......don't know what to do to solve it.
When trying to remove the old/original RH side sensor I could NOT budge it. I have the special O2 socket/tool that has a split for the wires and it started to round off the hex. So I had to cut the end off the sensor to use a 6 point long HD socket. Still couldn't unscrew it; so out came the torches. It started to come out but would freeze after 1/8-1/4 turn; thus keep reapplying heat. I brute forced it and it finally came out. But I must have applied too much heat and the threads of the sensor melted in the female hole in the down pipe (pic # 2). So next I had to remove the down pipe/cat converter. Upon removal I noticed the Cat honey comb was partially broken and melted inside (pic # 3). Same for the secondary Cat behind it. Fortunately I have the pipes from my parts car, The PO had gutted them, so I wanted to replace the LH side as well. I learned something some of you may not know. The down pipe has a honey comb in the "canister" portion of the converter. BUT it also has a "pre-cat" honey comb in the mouth of the down pipes right up where they bolt to the exhaust manifolds. (pic # 4) Now I had a dilemma with the LH pipe. It's a very arduous task to remove it due to the steering rack and heat shields etc. on a LHD car. I could/did chisel out the honey comb in the canister portion in situ (after removing the exhaust pipe behind it). I could not get to the upper "pre-cat" honey combs, So for now, the exhaust is not balanced but I don't think it will be noticable.
SIDE NOTE: Good news. Gutting the CATS have awakened the engine; it revs quicker, feels more responsive and more powerful. (Remember there a 4 honey comb sections; ea individual down pipe, the main Cat and the secondary Cat...plus 2 mufflers; on each bank---12 exhaust plugs in all)

$57 each Rock Auto.

Melted sensor hole threads.


Last edited by Fla Steve; 06-06-2019 at 09:07 PM.
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leo_denmark (06-07-2019)
Old 06-07-2019, 07:38 AM
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Just tossing out an idea....

Do a little experimenting with the idle mixture adjustment. Maybe one click in both directions? See if anything changes. I'm thinking the mixture might be just on the cusp of too rich or too lean....causing the feedback system to struggle a bit?

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Grant Francis (06-07-2019)
Old 06-07-2019, 08:57 AM
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Thanks Doug for the suggestion. This is a Marrelli ignition; so are you talking about an adjustment screw within the Marrelli ECU at the RH kick panel?
Or is there an adjustment screw within the Lucas Fueling ECU in the boot?
BTW: the pulsing is more noticeable now that the engine is breathing easier with the gutted Cats.
For backgroung: I've replaced the spark plugs; gapped at .025, HT leads, Dizzy cap and rotor, coils, set the butterflys at .002, cleaned the TBs, adjusted the TPS at .035V, . and put in a new temp gauge sender. Also new gaskets under the intake manifold pipes. The Smog pump is gone and the exhaust/injection rail tubes have been removed and holes plugged. The AAV is working as well, can get idle down to 700-750 rpm in gear after warm up: 800-850 in neutral.
I have Not replaced the other sensors i.e., Coolant temp sender for ECU (RH bank), or the air temp sensors for both the Ign ECU (bi-metal piece on RH air filter housing) and Fueling ECU (LH air horn). I haven't been able to find any vacuum leaks. What else could it be...aside from Doug's suggestion?

Last edited by Fla Steve; 06-07-2019 at 09:14 AM.
Old 06-07-2019, 09:08 AM
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If the throttle rods/plates are not 'synchronized' the engine will often 'hunt' at idle.

Old 06-07-2019, 09:37 AM
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Originally Posted by motorcarman View Post
If the throttle rods/plates are not 'synchronized' the engine will often 'hunt' at idle.

I set the butterflys at .002 each side and adjusted the link rods with just a tiny bit of slack. But I will try again. The other thing I have not done is to have the injectors professionally cleaned, or replaced the little pintal cap filters?

Any other ideas? Maybe there is a vacuum leak I just haven't found yet...but would that make it hunt if it's a steady leak?
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