Dear list members,

I hope you can offer some advice and point me in an as yet unexplored direction to fix a down on power issue with my 1999 P38 4.0L Range Rover.
I've owned the vehicle for the past 8 years as my daily user and now have 160,000 pretty much trouble free miles on the clock.

Symptom: Starts fine, runs a little rough, no backfiring, feels like it's not always running on all 8 cyls, needs a lot of gas to get it up hills, seems to run OK at highway speeds when not under load.

What I've done: New plugs, new plug wires. I cleaned and rebuilt the injectors (rigged up a test rig firing carb cleaner through them- new 'O' ring seals fitted), rented a 'Noid' light kit to check firing of injectors - all good, healthy spark coming from each (new) plug wire, rented OBDII - no codes stored, did a compression test on each cylinder: results (in psi) 138; 130; 153; 148; 156; 153; 154; 158 I conclude that there's no cylinder head gasket issues although I have had to top off the coolant in the header tank periodically. No obvious coolant leaks.

She just drives like a slug, feeling very underpowered. Situation developed gradually over say the past 5000 miles. Vehicle never really felt as spritely as my previous 2 (Classic) Range Rovers.

Appreciate help / comments.
Roger

 

clean MAF sensor when cold, with made for maf spray. Don't even turn on key, the sensor warms up inside to a high temp, and the cold spray can fail it. Clean throttle body butteryfly with carb cleaner, plus the PVC system and ports they enter on the throttle body. Fuel psi test at injector rail? Be sure TPS % is changing on scanner when you depress gas pedal.

My thoughts are that you have less fuel and/or air (or un-metered air, like a vac leak) than you need, but a sensor(s) is telling the ECU whatever lie it needs to hear to be happy and not spout codes. With scanner while underway see if vehicle speed is logical, as opposed to 100 mph plus. The VSS signal is monitored for high vehicle speed, the ECU then invokes a shut down / slow down routine.

 

The no codes thing makes this difficult to diagnose. Clogged converters might be a possibility also, but I would have expected there to be some codes with that as well. At 160,000 miles you probably have a pretty flat camshaft which will limt power also but it sounds like something more than that.

 

Thanks to Savannah Buzz and 04duxir for your input. I'm going to start on the list you suggest.

This morning I changed the fuel filter (I'm not going to tell you the last time I changed it!) just in case...but there's been no difference to the driving condition.

I'll rent a fuel rail gauge to test fuel pressure at the rail and report back.

quote: "Be sure the TPS% (throttle position sensor) is changing on scanner when you depress the gas pedal" I don't have a scanner - can I measure it's operation with a multimeter?

Thank you
Roger

 

It should just be a schrader valve. A tire pressure gauge is all that is needed.

 

Just completed the following tasks on the vehicle in response to list suggestions.

I bought a can of MAF cleaner spray, gave the sensor a good dousing. Liberal use of carb cleaner on the throttle butterfly and the shaft pivot either side of the butterfly. Although stained, it was functioning correctly.

Took a test drive to see if there was any difference... I was passed by a road grader (pulling a trailer). Guess that wasn't the problem but a couple more to cross off the list of possibles.

Loaned a fuel pressure tester from Autozone, and came up with the following:

Ignition on but not running - pump energized 38psi
Start up and idle, fuel regulator cut-in after about 4-5secs and settled at 32psi at idle. (manual says 34-37psi) - Q: Is this slightly low pressure an issue?
Q: Does the pressure increase at higher rpm?

(thanks for the tip on the Schrade valve tire pressure gauge but I'd already got the loaner tester)

In answer to one post regarding camshaft wear having 160k miles on the vehicle, last month I cured a coolant leak from the manifold to head joint, so I got a visual on the cam. Yes, you're correct (and thanks for pointing it out), there are signs of wear, but more light scuffing/ staining of the lobes rather than lobe rounding or worn edges / hardness issues. I didn't take any measurements.

And yes, the vehicle had the same sluggishness before and after the coolant leak repair.

I really appreciate the assistance, please keep it coming.
RogeR


x

 

In response to a previous post, the VSS (speed) sensor is apparently working as speedo indicated speeds / road speed appears correct.

 

have you tried dropping the exhaust yet?

 

Sorry, not familiar with term 'Dropping the exhaust' do you mean decarbonizing?

 

I am referring to disconnecting the exhaust at the manifolds to see if you have a plugged catalytic converter.