91 RRC with hesitation at 1800 and 2300 RPM

[lionvt]

91 Range Rover Classic. The car has a pronounced hesitation at 1800 RPM and 2300 RPM under throttle. The idle seems fine. The car does not stall. I checked for a vacuum leak or loose electrical. It happens when the car is cold, still happens when the car has warmed up and but goes away after about 20 minutes of driving. It seems to be worse on damp and cold days. My sense is that it is a mixture problem. Could it be oxygen sensor problem? These are the original sensors I think (125K on the vehicle). No codes have been thrown. Would love to hear some ideas on this one. Thanks.

 

[Gordo]

sounds like a tps to me. is the check engine light on? This should trip a code though as should the O2 sensor. easy enough to watch the voltage on while driving to test the tps. they tend to go out around 2500 rpms, especially if you drive a lot on the interstate. just a guess though. gordo

 

[lionvt]

No check engine light

No check engine light. The tps was replaced within the last two years so I doubt its that. Its worth a thought though. I'll double check the wiring to it as well tomorrow.

 

[Arthur 1995 LWB]

Test the TPS again. Mine was replaced 10K 1.5 years ago. Guess what, it failed. It acted up as temp varied between cold nights and warmer days during fall.

Ever replaced your coolant temp sensor. That would affect your mixture. At that, change your thermostat too. Perhaps VSS comes into play?

125K on original O2 sensors. I think they're past their life.

 

[nosivad_bor]

when my fuel pump was dying i had the same symptom. it couldn't produce enough pressure. that lead to the vehicle starving for fuel when i was around 1800 - 3200 rpm under load, like going up a hill or something,

check your fuel pressure. less than 32psi you are going to have issues.

 

[Nomar]

Will, my 91 Hunter has done this occasionally--significant hasitation/stall @2000ish rpm, may do it several times on a given day and then not for several weeks! No CEL, new cap/rotor , plugs, wires, coil---havent figured it out yet....still tryin'

 

[lionvt]

TPS testing

The fuel pump and filter are less than a year old so I (hopefully) can rule them out. How do you test the TPS? This vehicle is not tripping any codes.

 

[Gordo]

there is a multimeter voltage test in the workshop manual for my 89RRC. You can check the 'swing' of the voltage quite easily. It should be smooth with no noticable spikes/valleys. Check the EFI tests . Hopefully your workshop manual has them, if not PM me and I will send you the tests. Please include a return email. Still I think if the EFI light is out then more than likely it isnt the TPS or the o2 sensors. They would both set off the light/code more than likely. Gordo

 

[lionvt]

Coolant temperature sensor?

I am curious about the coolant temperature sensor. Where is this? Is it different from the sensor feeding the dash display? And if it is malfunctioning, wouldn't it cause a code to trip?

 

[Gore Ranger]

I have a '91 RRc too, and I replaced that last year when the truck was running hot. It turned out not to be the sensor after all, but it was cheap so it was no big deal. It is a little hard to get to without removing some things, but it can be done.

There are two sensors near the thermostat housing, each w/ little wires going to them. The two pronged one is the AC fan sensors. The one pronged one is the one you want, the coolant temp sensor.

If it was malfunctioning, you would just get a shitty reading on the dash guage, but then again, you might be anyway.

It looks like this...

 

[rrrandall]

If your idle is fine then there is vittually no possibility that the hesitation is ignition or vacuum leak related.

You can test if the hesitation is truly rpm related by driving the truck in 3rd gear. The transmision is degigned to freely shift back and forth between 3rd and 4th without even pushing in the button on the selector.

The O2 sensors are being ignored under accelleration, and the ECU determines fueling from the chip installed in the ECU. If you do not replace the O2 sensors at 100k, then there is uncertanty as to their performance when you have a problem. There are generic(has to be the same sensor) replacements which are easy to splice to the RRC connection, if you want to save $100

The VSS has nothing to do with this.

If this occurs when the engine has not fully warmed up, then there is a chance that the coolant temp sensor is a factor in the problem. The sensor shown in above photo is for the temp gage on the dash and has nothing to do with the hesitation. There is a 3rd temp sensor on the engine intake manifold, which is the one which modifies the fueling of the engine. This temp sensor looks just like the fuel temp sensor on the fuel rail. You should replace it at 100k also, and is cheap and easy to do.

If the engine is hesitating under accelleration then the TPS is definatly suspect. You may be able to clear the problem up by moving the trottle thru it's full range several times with the engine off. Otherwise the best way to test a TPS is by substitution.

There is one vacuum related device connected to the intake which is supposed to turn on at ~1800rpm. The ECU purges the gas fume canister when the engine reaches 1800. There is a wheel barrel load of hoses and connections in the gas vapor purge system. To make matters worse they changed it every year. Best thing to do is put a dowel pin in the hose that is connected to the intake manifold, cuttng off all that plumbing in the purge system from the engine. That may solve the problem or at leat eliminate the purge system as the problem.

The alternator is the only other thing that I can think of which is rpm related. If the alternator is flaking out around 2000rpm then there definatly coud be a problem with the fueling system. The ECU monitors the voltage and makes the needed adjustments. However if the voltage is out of range then the system could go a little wacky because all of the inputs/outputs could become inacurrate with the voltage swings. If it is the original alt, then you may want to replace with new now, rather than on the side of the road somewhere. The new ones are better than the original Lucas alt anyway.

Good luck
Randall

 

[lionvt]

Why would this work?

First of all, thanks for all the great ideas.

"You may be able to clear the problem up by moving the trottle thru it's full range several times with the engine off. "

Do you mean just bop the throttle linkage with the hood popped through its full range? And if so, why would this fix the sensor?

 

[rrrandall]

Just move the throttle linkage thru it's full range a few times(with your foot). I would not hold out a lot of hope that this will fix the problem, but it should help the TPS anyway. The TPS is really just a potentiometer, and pots have wipers built into them to keep the internals clean. Crud builds up on a pot if it stays in the same general position most of the time. Again...substitution is the quick and easy check.

 

[lionvt]

How do you adjust the TPS?

Here, and elsewhere, it has been suggested to adjust the TPS. I don't see how it could be adjusted. How do you do this? Thanks.

 

[Gordo]

hey bud

Here, and elsewhere, it has been suggested to adjust the TPS. I don't see how it could be adjusted. How do you do this? Thanks.

The 91 isnt adjustable only the 87-88s. Try to bum one off a buddies RRC or early 94-95 D1 to try it. Only a couple small screws and you wont hurt it. Gordo

 

[lionvt]

TPS replaced to no avail.

I replaced the TPS and no change.

But it gets stranger. Now desperate, I took the Rangie to a local specialty shop where they too suspected the TPS. I explained that it was only a day old. They said they tested it and it was faulty. So they replaced it (with my blessing).

New TPS (now the 3rd) -- and still the hesitation.

So, is something frying the TPS (ideas)? or is it something else.

Also, the hesitation is now more dramatic but more intermittent. When it is there, it is definitely there. But sometimes it runs fine. Actually, it seems to run fine in cold (0 degree F) but not so well a 40+ F, but that might just be a coincidence.

 

[sven]

Try removing the distributor cap and check the mechanical advance. Also check the vacuum advance. Could all be seized up.

 

[lionvt]

Still puzzling over this one but I wanted to thank every one who has chimed in. I really appreciate the time people of spent coming up with ideas.

The only other thing I can add is that the hesitation is intermittent and seems to be a problem on some days and not on others. It is about a 30 minute drive to town. Yesterday it hesitated the whole way. Today it was fine. Which of course makes me think electrical problem somewhere.

I brought it to Autozone and they hooked up the battery/alternator tester and the alternator seemed fine and the battery somewhat borderline. I replaced the battery as a precaution but no change.

 

[Gordo]

Hey Will. I know how frustrating this can be. You didnt mention but have you replaced the cap/rotor? How old is the amp module? This one is really puzzling me too. Maybe your mechanic or someone here can chime in about vacuum testing the motor. It could be a clogged cat or something. Ive done it before on mine but dont remember the figures. When it is hesitating get out and check the cats. If they are glowing, that could be why. Also run the injector voltage test at the ECu in the early workshop manual. Probably a good idea to run all the 'easy' voltmeter tests while your at it. (if you dont have it PM me your email and I can get it for you.) Maybe you have a faulty injector or something. Just guessing but offering up some options. Any other noises, popping intake, exhaust, etc? One thing for certain, I would stop throwing parts at it to try and fix it. Find the problem and fix whats broke. Gordo