Spluttering/hesitation issue on light throttle

On my 1993 Jaguar xj6 Vander plas it’s inside the tank. I had to rewire the hole pump and change the connection because I couldn’t find the old ones. While I was in the tank I checked the inside for rust or anything that might be a problem. I’m not a expert at this but I’ve been around cars all my life and had to learn how to fix them on my own. With that said it’s not a hard car to work on. I enjoy working on my car.

Thanks for the tip, the fuel filter was the wrong one so I couldn’t change it. Correct one ordered now (hopefully).

I’m used to working (a lot) on older cars with carburettors, so the fuel injection system in the Jag is a little unfamiliar to me. Likewise, I’m not familiar with automatic gearboxes, as all of my other cars are/have been manuals. Learning though, and hopefully this is something simple!

The issue seems to have “moved” slightly after changing the rotor arm. Previously in “D” the issue would present itself at 50mph, it’s now happening at 70mph… Could it be ignition related, or perhaps coincidence… I did hear of the crank pulley woodruff keys destroying themselves which causes the timing to move, but I’m not sure how that could be load related.

Be careful removing the old fuel filter, the unions are usually rusted solid if it hasn’t been changed regularly and the metal parts of the fuel pipe can twist and snap, I’d drench them in penetrating oil while you are waiting for the replacement filter to arrive so it has time to seep into the threads.
Are the HT leads all in good condition ? If they are old and beginning to go hard and crack the spark can ‘track’ across from one lead to another under load causing the symptoms you describe, you might be able to see this if you rev the engine when it’s dark but even if you can’t I wouldn’t rule a bad lead out, also when were the plugs last changed ? A badly worn or incorrectly gapped spark plug can cause misfiring under load.

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I actually loosened them off before realising the filter was wrong. They were caked in grease and came off pretty easily!

Interesting on HT leads and spark plugs. I had intend to change them very soon (and potentially as part of this), because they do look old.

Well, I think the Jag is fixed.

I drove to my workshop today to change the fuel filter and noted that the issue had gone away.

I suspect this was perhaps a transmission issue afterall. I hadn’t driven the car a great deal since changing the transmission fluid and maybe it hadn’t had time to circulate/mix with the remaining fluid?

I think changing the fluid again will be a good idea.

Massively appreciate all of the replies and help offered, you guys are great. :slightly_smiling_face:

Well, the optimism was short lived. I drove it to work on Monday and the problem appeared again.

I decided to unplug the MAF and drive it. The car drives 95% better with this unplugged. I plugged it back in and it instantly returns to the hesitation issue.

I say 95% as the problem was still very slightly there but significantly reduced.

Before I purchase another MAF, does this sound like the issue? (I’m concerned an unplugged MAF may enrichen the mixture and mask something else causing a lean condition). At any rate this is progress and definitely points to it being a fuel injection related issue.

Did you pick up a temperature sensor yet? These are very inexpensive and can affect the running of the car dramatically, just like in your case.

I once bought a defective one and my car ran like $hit.

Oh and I’m talking about the primary temperature sensor, the one located near the Tstat housing and next to the temperature sender.

I’d went down a transmission rabbit hole so not yet - but it has been on my list!

This one?

Yep. Aftermarket ones out there too.

I just ordered a Lucas branded one, thanks! You are right that at such a low price, it makes sense to rule it out.

To add, I unplugged the Lambda sensor and that didn’t seem to help, so I’m ruling that out.

That’s interesting …it’s the primary controller for enrichment and it made no difference? You must have got a CEL from that though, eh?

Some research today pointed me in the direction of the Lambda sensor, based on the symptoms… No CEL from it, which I thought was odd.

I say it didn’t change anything, the car did run a little differently (revs tend to hang a little while), but the stumble was still there.

I think if the Lambda sensor was causing the problem you would definitely get a CEL and a code to check. The parameters are quite small and anything outside of them is picked up quickly by the EMS.

I don’t think the transmission oil would cause spluttering and engine hesitation, even though it’s impossible to completely drain the system the fresh trans oil would have mixed completely with the old stuff within the first few hundred yards of driving.

Was there a big difference in the weather on those two days ? On the day you thought it ‘had gone away’ was it a dry day and raining on the day it re-appeared, I’m only asking because perished / cracked HT leads or a cracked distributor cap would react a lot worse on a damp day.

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The strange thing is that when I unplugged the Lambda sensor, there was no CEL code?

The theory on the transmission was that since it was occuring at >50mph, that it could be the torque converter lock engaging/disengaging. I’m not too familiar with automatic transmissions, so it was a little bit of a spanner in the works. Confident it’s engine related now though.

I drove it to work all week this week, to try to get more of a feel of what is happening. A mix of wet and dry days but the problem was pretty constant.

HT leads were high on the suspect list until I unplugged the MAF, which seems to pretty much cure the issue - so I suspect it is either the MAF or perhaps another fuel system sensor which is being masked by the car returning to “default” fuelling with no MAF?

Fuel economy is significantly worse with no MAF, so I assume the car is running richer without it. I think I can rule out the fuel pump from this also.

It’s difficult to diagnose the problem you are describing without driving the car, but from your explanation and having owned and regularly driven an XJ40 for the past 20 years my gut feeling is you have an intermittent problem in the ignition, either a failing spark plug or damaged HT lead or both. The EMS is quite sensitive and will throw up a CEL and code if any of the critical components such as the MAF or Lambda sensor fail or are failing. Reading your posts you have NO CEL or codes whatsoever so that suggests it is neither of those parts or even EMS related. In your position I would remove and examine the plugs comparing each one to see if one is noticeably sootier / oilier or stands out for any reason. Better still would be to replace all six along with the leads and a new distributor cap and rotor arm. These parts are considered as ‘consumables’ anyway so even if the fault remains at least you can rule them out and you will have also completed a necessary service recommended in the Jaguar manual.

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It is always difficult to describe issues like this without driving the car, like you say!

I’ve replaced the plugs and rotor arm already, so I’ll also order a set of leads/rotor cap and see how that goes - neither of them look particularly new, so it would be good to replace regardless.

I changed the HT leads, distributor cap and CTS but the problem persists. Unplugging the MAF still cures it, so I think I need to try another MAF despite the fact that there is no CEL?

Well, I still haven’t fixed this which is great!

A new sympton has appeared though…

It won’t rev above 3500rpm now. When it kicks down, it accelerates up to to 3500pm, gets stuck and won’t change gear. I can only assume this is related to the hesitation issue.

Potentially a clogged catalytic converter? Could explain why there’s no CEL?

Or the dreaded fuel pump?

Still suffering this issue, the car has been parked up for a few months as a result.

Just bumping this up incase anyone has a light bulb moment, or something.

Have changed:

MAF
TPS
Ht leads
Rotor arm
Distributor cap
Fuel filter

I’m thinking of next looking at the ignition coil, and then the Lamda sensor.

Interestingly someone on the XJ40 facebook group is experiencing the same symptoms, but also doesn’t know what the issue is.

Change the coolant sensor. Sometimes these things can really mess up performance. I replaced one with a defective unit and my car behaved like yours.