Brief lumpy idle after highway drive (but only in park)

Just had a great drive in my '89 XJ-S today. About 100 miles of interstate to stretch the cat’s legs! My car is running very well and ate up the pavement at 75MPH+!!

I noted that right after I got of the highway and arrived home and put it in park it started idling a bit lumpy between maybe 800 and 1000 RPM. I put it back in drive and the idle smoothed out immediately. Then I left it in park running and within 2 or 3 minutes it was idling smoothly there too.

Any ideas what caused a lumpy idle after a highway speed drive but only when the car was in park and only for a few minutes at that? Thanks for any ideas!

I will note I never have any idle troubles any other time. It starts and idles smooth at about 1100RPM when cold and once warmed up automatically drops to a ~750RPM smooth idle. Also, it’s fairly cold here today, think I experienced this at about 28F outside.

One word Lucas :joy::joy::joy::joy::joy::joy::joy: 20

1989 could be Lucas, but could very well be Marelli :-))))

Sorry, not good ideas. Usually a nice highway drive clears the internals a bit and engine runs “better” afterwards.

You are reporting idle that was slightly higher than normal idle speed, which would be suggestive of extra air getting in. AAV or SAV? I don’t think either can be responsible based on the fact that after you put in in drive, all went back to normal.

How are your throttle linkage bushings?

Very odd. Is it misfire? Only difference between P and D is that it uses open loop fuel map in P.

Perhaps engine is not running as well as you think, and when putting it in D, it now uses your ecu fuel mixture which you may have dialed up extra rich to compensate?

But there would be a difference in the load on the engine when in drive.

Lucas ignition (early '89 built 10/88). Throttle linkage bushings are good–just took a look.

Misfire is an interesting idea Greg–I recall your troubles with a flaky injector that gave you troubles some of the time. I have not cleaned or had the injectors cleaned on the car during my ownership (so that’s certainly on my list). The basic bill/paper behind the exhaust test does not suggest a misfire on either bank. It also has plenty of power going down the road (though I imagine this engine still would even if on 11 or even only 10).

I have not dialed up the ECU mixture ever–I have seen discussion of you and others doing that but to be honest have never tinkered with it myself. I will investigate to determine what click it is currently at.

I will also look around for vacuum leaks–I have been doing many projects under the hood–not out of the question I knocked something loose and didn’t notice it!

Oh and last thing–I think I must be running a bit rich as my MPGs are not as good as what some of you report. I seem to get 13-15 highway and about 9-11 city. Not terrible but I imagine some tuning, clean out, and adjusting would improve those numbers.

Engine load in drive will undoubtedly change the rpms and the way the engine runs, but the OP reports that after the highway run, in P, idle speed was 800-1000 rpms.
Then in D, all was well, and then back to P - no problems.

I am not sure what can cause such behavior, misfire is unlikely IMO.
Since the car has mechanical ignition advance/retard system, perhaps this is where the attention should be focused.

Been thinking about this, and i may get this too.

But mine is when i shutdown engine, and then restart it about 10 minutes later. Lumpy idle in P which eventually smooths itself out in a minute or two.

I attribute mine to fuel in the rail getting a bit hot due to heat soak? I do not have a fuel cooler anymore (removed ac) nor do i have vacuum valve that increases fuel pressure when it senses hot fuel in rail.

Could be similar issue?

I have often experienced this over my 25+ years of ownership. I can’t say what really causes it but I can say that it is temperature related and temperature soak related. It has always occurred when the auxiliary fan is mal-functioning or insufficiently providing cooling during running and after shutting down during a prolonged run.

I would also say that my description is not ‘a lumpy idle’ but no idle at all after a start up and having to coax it into an idle which is lumpy until I can coax it into not dying and keeping it running until I can get a throttle response that actually causes more rpms from throttle response while moving the throttle as if it needs to be pumped although that is not really what is happening or maybe it is happening via the vacuum change enrichment feature causing fuel enrichment. The system is not a Venturi carburetor set up with an accelerator gas pump in the carb.

At any rate, if the auxiliary fan is working fine and there is a standard cool down period after shut-down, typically for 5mins or so the problem of an easy start and lumpy, very poor idle after a long run at upper speeds or a hot weather run completely disappears.

I was experiencing that last week and after inspecting every thing, found the plastic connector on the thermostat at the water pump in my hand in pieces and that the auxiliary fan was not operating or operating intermittently.

I will only know the answer when the weather clears up. The Jag is inoperable presently because of snow and possibly no available time due to beer and football weekends. I just did the discovery and repair on Friday. Beer and football on Sunday with snow Sunday night and no extracurricular travel because of bad weather and snow conditions on Monday. This has been the remedy to this problem in the past, several times. I will let you know when I have validated the no auxiliary fan operation fix as solving the problem. I hope I don’t also develop ‘Cabin Fever’ from not being able to get out and about.


Mine has the same no idle after starting after a good freeway run; I have upgraded the electric fan, have a new radiator, and a new clutch/black mechanical fan. It never runs hot, and the after run fan only comes on if it’s extremely hot out.
It has something to do with fuel pressure as when it happens, I can hear gurgling noises from the fuel tank. My money is on one of the regulators allowing the pressure to bleed off in the rail, probably the inlet one as my fuel pressure running is right on spec and I have a functional thermo switch to the left regulator.

I run 180F thermostats, and it’s almost never hot enough here in Western Washington for the aux fan to come on, so my heat soak engine temps do not get that bad. But I still get a bit of lumpy idle after a 5-10 minute heat soak. It starts up no problem, and I don’t need to give it any gas to keep it going. But it idles about 100-150RPM less than normal.

BTW, I tested vacuum to vacuum advance while it idles a bit lumpy, and it was a steady 10 inHG. So it doesn’t seem timing related, at least in my case.

I’ve read a cure for this is to simply cycle the fuel pump a couple times before starting, to help flush out any fuel that may be too hot.

I also noticed during my lumpy idle, the engine vacuum which is usually 18inHG was only 17inHG. I wonder, if after a shutdown or highway driving, a lot of fuel vapor has built up in the charcoal canister system from the heated fuel in the tank, and the engine is trying to rid the system of that pressure/fuel vapor. It feeds right into the intake system. And it clears up in a minute or so because it’s bled off that pressure/vapor?

My 88 had the recall upgrade to the fuel vapor system so that it cannot get into intake manifold when engine is off. So perhaps that’s why I can easily start? When you guys say you have a difficult start, have you had the recall upgrade installed?

It IS a super complicated system. Mine has three vacuum lines (two from top of throttle body, one from intake manifold) and then the hose to send vapor from canister to intake manifold for burning. One day I will study it and make sense of it, but for now, I’ve just replaced all vacuum lines and the Rochester valve.

Anything that lowers idle will generally lower vacuum. You can deliberately raise your idle to 1500 – even just by holding the throttle open a bit – and watch the vacuum increase.

So i did some investigtion, and noticed a slightly lumpy idle after driving only in P, not in D.

Idling in Drive, dollar bill test on exhaust proved perfectly smooth idle. No misfire whatsover.

Idling in Park, B bank was still smooth, but A bank now had intermittent random misfire. (1-2 seconds smooth followed by 1-3 misfires in a row) Trying to raise/lower idle with idle screw made no difference.
Openingg/closing A bank throttle clearance made no difference. But the idle in P did smooth out when i forced a richer mixture.

Now, I’ve been doing only short 20 minute city drives for the last 5 days which may explain mine. Which is opposite to you.

That’s odd! Yeah for me I have only noticed this occurring after highway runs. And in fact, I’ve only done small around town drives in the past few days and have distinctly noticed it’s not occurring when I put in park after 10-15 minutes of city driving.

I looked around very carefully and do not think I have any vacuum leaks from recent maintenance. The vacuum vent line to the distributor cap was maybe not seated all the way but I don’t think it was leaking or that its position was anything new; in other words, the fact that it was not fully seated down on its barb as far as it could be has probably been harmlessly the case for a while. Furthermore when warm and stably idling (i.e. not right after a highway run) there’s nice stable vacuum of ~19inHG. That seems perfectly acceptable to me from what I’ve read on here.

I think your all’s points about fuel pressure being allowed to bleed off are interesting and make a lot of sense as the explanation here. One of my first projects when I bought the car was going through the whole fuel and fuel vapor systems. So I learned a lot about that system at the time. I will take a look at the canister and valve and all and make sure everything is still operating as it’s supposed to be but I’m pretty confident that system is working alright (to the extent I understand it because, Greg, I agree it sure is complex!). And happily I have never had heat soak/hot start trouble getting the car started. If anything it starts more readily when fully up to temp than when it’s cold from the night before.

It certainly gets hot enough in the DC area where I am to require the aux fan in the summer time–but I haven’t noticed this lumpy idle in hot weather, only in cold. But then again if it’s a new development then it might well occur when it’s hot too. I will definitely keep an eye out for this out of interest sake but I feel fairly confident it’s not a sign of anything seriously wrong that needs immediate attention!

Anyhow, there’s supposedly 6 inches plus of snow on the way here next weekend! Question is weather to take the car for a spin (potentially literally) in the snow! It’s my daily driver so it gets 12 months use and I just be sure to give it frequent washes when the roads are salted. I got “brined” by a deicing truck on the beltway last weekend…but turns out you can wash your car, undercarriage and all in the driveway when it’s 35F out–just requires a lot of layers, really good gloves, and working fast!! Your year round mild climate sounds pretty nice right now!!

I would not go out with salted roads! Poor Jaguar…

I do NOT like driving my xj-s if the roads are even wet, let alone snow. All that rear wheel drive torque along with limited slip, i have to baby it when accelerating. Too many times I’ve had the rear end decide it wants to be free if i give it too much gas.

And our idle issues sound very different, although both could still be a lean mixture. I just think the closed loop o2 feedback helps it adjust a better fuel mixture in D.

On the other hand, as my daily it gets all my attention and when something goes wrong I turn to it immediately! So it has to brave the winter true, but it gets the attention (and finances ha!) that might otherwise go into other projects! Fortunately salted roads and snow happen at most half a dozen times a winter around here!

I put new tires on the car earlier this month–big difference in all driving conditions those made! The P600s that were on it were too old to be driven on safely even if their tread looked decent.

And yeah, I agree that our issues seem different. I will keep observing and report back any further developments I notice!

I also forgot, one thing that could be affecting my idle a bit in open loop, is that I have advanced the timing from 18 to 22BTC (@3000RPM). (I switched from Regular 87 octane to Super 92 octane). So my timing at idle is also a bit more advanced than normal. That is giving the fuel less time to ignite which ‘could’ be contributing to my issue. I assume in open loop, the ECU is going by a fuel map and has to assume the timing is where it thinks it is? These cars are the strange time in between carburerated engines where you could mess with this stuff, and fuel injected engines with an ECU that knows exactly what the timing is. (Did this start with Marelli engines?)

Is your timing at 18 BTC?

Interesting thought! Yeah mine is at 18BTC. I might try advancing the timing a bit as you did but I haven’t yet. I also run high octane in my car (93 is what we have around here).

Advancing the timing definitely gives it a bit more pep/power.

My USA 1988 manual says to use 91 Octane RON. But the RON is a European rating. Translated to American octane rating (AKI), that is 87 Octane AKI. That is regular. So you don’t need the Premium 93 at all if you’re running the standard 18BTC.

Roger Bywater stated that 1 octane boost allows 1.5 degrees of advance. Those are probably RON. So I went 4 degrees advance with an AKI octane increase of 5, well within the limits recommended.

I found from a table: 91RON=87AKI 96RON=92AKI 97RON=93AKI