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Old 06-03-2018
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Question Bad hesitation after driving for 20-30 mins (thinking heat soak?)

Hey guys,

Long time no see. Haven't been driving the XJ for a while as times have been tough and haven't been able to keep all the cars registered, but such is life... She's back on the road now and I'm remembering why I have always loved driving the old girl

Anyway, I've had this problem for some time now, and I just want to run this by you guys to get your opinion too, to be sure. My XJ (95, 290k kms) starts instantly, idles great, drives great. Basically perfect. After I've been driving for about 20 or 30 mins, sometimes more, I get this hesitation that starts minor and gradually gets really bad - like someone was hitting you from behind. There is a corresponding little drop in RPM in each instant it does it and you can hear an audible change in tone from the fuel pump - I have no interior so it's loud as inside, I hear everything.. I'm not sure if it's actually heat soak, because I can stop the car and start it again and it never really has a misfire as such, just every few seconds it does this sudden hard bog down and recover - sometimes it does it repeatedly 3 or 4 times, almost feels like it wants to stall out. Injectors and fuel pump are all original, it started out of the blue a few months back. When I leave the car sit overnight, the next day it's back to normal and the cycle starts again.

I'm surprised if it's heat soak because I have put this XJ through some real tough heated situations in the past and it's never done anything like this. And just recently I ripped out all the A/C gear since that hasn't worked in over 10 years. Since taking out that A/C radiator and such, the operating temps have been AMAZING, I wish I had done it sooner. Instead of always going to that 100c+ mark and beyond, it's almost always between 70 and 80c and it never needs to turn on the aux fan... but I often do that anyway because I have the A/C bypassed and I still like to get some vent air in this humidity!! . With the fan on, the temp is pegged at 70c almost constantly, it's beautiful. But even if I have it on from the get go, 20/30/40 mins later, this horrible hesitation. Can try and get a video if you'd like? No error codes, no check engine, nothing.

Spark plugs, leads, map sensor, TPS, IAC, o2 sensor, water pump, thermostat, air filter - all replaced and new within the last 12-24 months. So my thoughts are on the fuel system, either tired old injectors feeling the strain now when they get hot, or a fuel pump starting to go but only showing when everything is driven and heated up. The hesitation happens regardless of speed or gear, although you really notice it when sitting still or just rolling along... more speed & more rpm's just kind of overpowers it a bit I suppose but I hate that feeling.

Sorry for the rant, I'm nothing if not thorough!! Really appreciate any insight, thanks

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Old 06-03-2018
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can't comment on the fuel pump noise part.
thing is the faster it goes the more fuel it needs, To my mind, if its a fuel supply issue faster should cause it to get worse not better.
Heating up then playing up, suspect: crank sensor, coil, cam sensor. in that order.
this usually gets worse till the car just stops, then when it cools down it starts again.
With those particular symptoms I would be eyeballing the coil.
Try dumping a liter of water over the crank sensor/ then the coil see if the symptoms go away temporarily.
good luck with it
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Old 06-03-2018
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I agree with rainman
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Old 06-03-2018
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Quote:
Originally Posted by rainman View Post
can't comment on the fuel pump noise part.
thing is the faster it goes the more fuel it needs, To my mind, if its a fuel supply issue faster should cause it to get worse not better.
Heating up then playing up, suspect: crank sensor, coil, cam sensor. in that order.
this usually gets worse till the car just stops, then when it cools down it starts again.
With those particular symptoms I would be eyeballing the coil.
Try dumping a liter of water over the crank sensor/ then the coil see if the symptoms go away temporarily.
good luck with it
Interesting, I hadn't really considered the coil or cam/crank sensors. What happens to the coil when hot that would make it do this? Just curious.

Crank sensor I replaced maybe 10 or 15k kilometers ago, so I imagine that should still be good. However the camshaft position sensor and coil I couldn't tell you about - if you asked me I would say they are original, we we've had the car since new and I cannot ever recall either of these items having being serviced.

So it's generally pretty safe to pour water over the coil? Happy to try that next time it acts up, no worries
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Old 06-03-2018
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My jeep ran like garbage with a bad Crank Pos Sensor. itd hesitate on cold starts, then 15 minutes later would turn off.... cold water down the back would fix it back up.

Im sure water over the dizzy would be okay..... as long as its not submerged (i think the cam sensors in here)
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Old 06-03-2018
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I had those same issues when my crank sensor went bad, and it always threw codes too thankfully. Never did try the cold water trick though!!

I'll give the water a try on the dist, doesn't bother me to at least try And yeah from what I understand that's where the cam sensor rests - I'm thinking in a few weeks when some money comes through I may just replace this along with the coil and maybe fuel pump. At 290k kms they're all pretty much end of life anyway, better safe than sorry.

This is all great info, this is why I wanted to start this thread. It's easy to just go 'heat soak heat soak!' but all I've done is IMPROVE engine bay airflow and cooling dramatically, so if anything it should be LESS likely to happen. We'll see!
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Old 07-03-2018
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There's a pretty big crack in the plastic housing of the ignition coil... Don't think I'll be pouring any water in or around that area. I'll have to start with replacing that and then go from there, see what happens. I can't say I've ever heard of an ignition coil that only acts up when it's hot (have you?), but regardless that's where I have to start before I investigate much else. Does anyone know if our coils have any date codes stamped on them anywhere? Be curious to know if it's original
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Old 07-03-2018
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Quote:
Originally Posted by Sobek View Post
There's a pretty big crack in the plastic housing of the ignition coil... Don't think I'll be pouring any water in or around that area. I'll have to start with replacing that and then go from there, see what happens. I can't say I've ever heard of an ignition coil that only acts up when it's hot (have you?), but regardless that's where I have to start before I investigate much else. Does anyone know if our coils have any date codes stamped on them anywhere? Be curious to know if it's original
Coils generally play up when hot.
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