P38A Down on power when warmed up

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enjoi

enjoi

Active Member
Posts
74
Location
Australia
Hello there!

I've got a 2001 4.0 P38 and I noticed over the last few months (I don't drive very often) that the ole gal has been down on power, especially under load, last time towing my boat (Only about 1 tonne) was yikers.
Turned out I had an O2 sensor error, so I replaced the both sensors and noticed a massive difference immediately, however, this new found power only lasted for a few minutes until the engine was at operating temperature. Once at around 91-94 degrees, it seems really sluggish again. It's not too bad on flat ground, but if any extra power is required for a small hill or getting off the line, it doesn't feel right.

I've had cooling issues in the past, but, from what I can tell, using my OBD reader, the coolant temp seems okay, usually sits around 94 degrees and when I'm moving it drops to around 91. So I'm not sure it's the ECU limping the engine to stay cool.

Not really sure where to start with the investigation, any pointers are greatly appreciated :)
 
enjoi said:
Yeah it is in a plastic housing. I replaced it about 14 months ago. What's the thinking behind removing the thermostat?
Click to expand...
It was to test and see if it was jammed open but if it's encased in the plastic I don't know how you would. 💁 I've not been there yet🤔
 
If the engine gets to 91-94 then the thermostat is probably working ok. If it was stuck closed it would overheat. Stuck open it would be much cooler.

The adaptive values might be way out & need resetting ? What does your reader say for long term fuel trims ?
 
Just did a bit of a test drive with the reader on. Long term fuel trim values seemed all over the place when I first turned the car on and it was getting to temp, like -1% 2% etc. Then it settled at around -6-7.5%

I haven't tried unplugging the MAF, but some readings in neutral;
Idle: 8g/sec
2krpm: 18g/sec
3krpm: 30g/sec

When driving it sat around 10-20g/sec unless I went up a slight incline, even below 2krpm it shot up to over 50g/sec
 
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Does your diag reader do recordings? Might be better to see graphs with o2, maf, fuel trims, rpm etc at same time.

Is there anyone local with a nanocom ?
 
IMG_1370.jpeg


Turns out it does do recordings! Went for a pretty decent drive today. Apparently averaged around 15L/100km which is pretty good :0 Didn’t buy the car for fuel economy though, still felt sluggish.
 
What speed ? Also did you reset the trip before the drive.

I always thought my old 4.6 was fine, until the day I got my project P38 4.0 on the road & MOT done. Both average the same MPG (16-18 around town & 21 on a motorway), but the project car with rebuilt engine & new camshaft is significantly more spritely. I also did piston rings & all bearings. [EDIT] Compression was about 140-160 before, and 180 after, but that was done with engine cold.

Your data above looks ok to me, so I would suggest measuring the valve lift with a dial gauge. If the cam is worn it will be slower & down on power. Default lift values are in RAVE specs.
 
Last edited:
worn inlet cam means it will run rich, down on power. might show as low compression
worn exhaust cam means it cant exhaust properly or possibly exhaust to the inlet, meaning with will run very rich and down on power
plug colour will be an indicator, depending how badly worn
 
FWIW, the engine was rebuilt by some fairly reputable folk about 70,xxx kms ago.

I didn’t reset the trip, however I did put my spare battery in because it was flat :vb-embarassed-laugh: which I believe resets the trip.

I’ll attach the vehicle speeds of the trip and in the second image I’ve just zoomed in so the O2 sensor values can be more easily seen.

IMG_1410.jpeg



IMG_1411.jpeg


Diving into the engine isn’t something I’ve ever dabbled in. I’m sure I can take a squizz at the plugs, not sure when they were last changed.
 
They look ok but I'm no expert on the tech side of things.
70k miles is an innings for an rv8 depending on how it's looked after. Have you pulled the plugs yet to check colour? Sorry I've forgotten
 
Definitely reset the trip comp ideally during the 50kph part. Then it will give a better average for the run.

On mine if I leave it at the "round town" average, then on the motorway it takes ages to achieve sensible readings. Not sure how long the P38 "average" is calculated, but if it's from "last reset", then you will never see the motorway consumption, just the overall combined.

Also try moving the seat back one notch & wear lighter shoes !!
 
pwood999 said:
Definitely reset the trip comp ideally during the 50kph part. Then it will give a better average for the run.

On mine if I leave it at the "round town" average, then on the motorway it takes ages to achieve sensible readings. Not sure how long the P38 "average" is calculated, but if it's from "last reset", then you will never see the motorway consumption, just the overall combined.

Also try moving the seat back one notch & wear lighter shoes !!
Click to expand...
I reset trip 2 at every fill, it then computes the consumption fairly quickly, within 30km.
 
Hi,

The first thing that came to mind was the fuel injection coolant temperature sensor. Maybe it's telling the ECU that the engine is still cold.
They are cheap to buy and easy to fit - just screw in.
Try a new one before doing anything else

They pretty regularly need replacement on older cars of all makes.

I had a go at finding the part so check out the screenshot from Jaguar/Land Rover Classic Parts, item 13 in the schematic
Good luck.
Clive
 

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Rubberknees said:
Just a thought Have you tried unplugging the MAF sensor and seeing how it runs.
Click to expand...
I just tried this and interestingly enough, unplugging the MAF seemed to make no difference. The idle was the same, and from my best judgement, the performance from a quick drive seemed the same.
I would have expected a super rough idle with an unplugged MAF? Do I have a dud MAF then?
 
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