Hello,

My '84 XJ6 has developed a rough, stumbling, low RPM (500 or so) idle but only in Reverse. In Park, Neutral and all forward Drive speeds, it's smooth but in Reverse, it feels like the engine is about to die. The previous owner took great pains to smooth out the idle, which he did. The car is fitted with a Pertronix Flame Thrower ignition system, iridium spark plugs and high performance ignition wires.

Any idea what could could be causing this?

Thanks in advance,
Rhett

 

Got me stumped, but it seems to me that when in Reverse there is more drag on the engine than in the other gears. Not being an auto transmission specialist, I'll have to duck out now and see if somebody more knowledgeable than me will post the right answer.

When auto transmissions are "in gear" the car will creep forward or back as the torque converter connects the engine to the gearbox. Modern cars with ECUs and inputs from transmission controllers (TCMs), compensate the idle for drag when in gear, (and also usually lift off the throttle when changing gear too). Older cars with transmissions like the BW66 don't have this.

 

Sorry, I should have been more clear. This car has been my daily driver for 3 years. The strangeness in the idle is new. I only cited the previous owner's efforts as a way of mentioning that there's non-standard equipment in the mix and to preempt any comments to the effect of "Series 3 idles are always rough". Because on mine, the idle is usually very smooth. The car is so consistent that it catches me a bit off guard when thing do occasionally, and inevitably, break. The BW66 is still in the car and delivering its consistent if rather lack luster performance.

 

Thanks for that, but I'm still stumped ! Maybe Doug who posts on here has the answer or maybe a suggestion.

I still think that transmission drag in Reverse greater than in the other gears is maybe what is causing the problem; what else could it be, as there is no link back from the transmission to the fuel injection system ?

 

My vote is on the "extra drag in reverse" theory. So:


1. As long as it does not die, just live with it. Little driving in reverse by most of us. Except possibly a pain when parallel parking.


2. Bump the idle up to about 600. You may get some creep at stop lights.


3. Seemingly, the XJ engine is not at it's best at idle. Some feature a means of idle speed increase to overcome compressor drag when the AC is on.


Carl

 

I'm stumped.

Must be some sort of strange transmission problem?

Or, since engine torque is resisted in the opposite direction when reverse is engaged perhaps some wires or connections related to ignition or fuel injection are being tugged or jiggled in just the wrong way? I'm really stretching here.

Cheers
DD

 

Darn. Thanks for trying.

 

Rhett:


I think Doug may be on to something. GROUNDS!!!


I cured a similar stumble in my F150 decades ago. It was intermitant there in all gears. I carefully wiggled wires under the hood, til it stumbled. Aha, a ground harness off the - battery post.


I eliminated a seemingly unnecessary connector and doubled the wire in black. Gone, smooth as a 302 can be.


Harder to do in your case, but, suppose the engine moves on it's mounts just a tad differently in reverse than it does when in a forward gear.


1. The "packet" of Grounds Doug mentions at the left rear of the engine. ON general principles, re do them.


2. Down and under. A ground strap on the right front side of the transmission connect to the chassis. Plenty of grunge there to mess up the contacts and/or the strap itself. I have a bias against those woven wire straps. I prefer the "normal" cables.


Now, if I can only get my LT1 to stop smoking to please the SMOG guy. Some progress, although. Dirty air cleaner. May be a weather delay...


Carl


Carl

 

My 83’ idles rough on Drive. I usually just bump it to neutral at stop lights. Or ride it out, gives the car character

 

Fraser Mitchell and JagCad you win the award! Since I posted the original message, the idle and overall running condition of the car when warm degraded to the point where I was sure it wouldn't momentarily self correct the moment my mechanic got a look at it (because these cars are like pets when they get to the vet!). A smoke test revealed a large crack in the shroud between the air intake and the mass air flow sensor housing, underneath where I wouldn't see it unless I knew to look for it.

Anyway, thanks for your theories; they helped me at least direct my thinking toward things like vacuum leaks and air intake rather than transmission problems. And, aside from my pesky fog lights not working correctly, the car is running great again.