Rough running - related to vacuum but how?

[lionvt]

91 Range Rover Classic. Here is a puzzler but I'm zeroing in on it. For months we have had an erratic rough running -- always at about 1800 rpm. Idles fine. Frustratingly, it is intermittent. Would be fine for weeks and then appear and disappear several times a day. Replaced the TPS. Have not been able to track it to any thing until now. I finally noticed on a road trip this weekend -- it happens when the climate control lever is switched over to the left -- the outside air/ac positions. This is why it seemed to come and go. I replaced the vacuum hoses some weeks ago trying to fix this so I think the obvious lines are good and not leaking. I know that the climate control flap (I'm not sure what it is called so I'll just call it the "flap") is vacuum actuated on this model -- so I know (I think) that my problem is related to vacuum. But, dear readers, how? I know the vacuum is working on this flap (or whatever it is) because I heard it snapping open/shut when I replaced the vacuum lines. So...do I have another leak somewhere that only manifests itself when the flap is in the shut (or open?) position? What do you think?

 

[ptschram]

Maybe the vacuum solenoid leaks when you open the valve to draw it open.

You have two choices. Disconnect the vacuum line and live with less control over the HeVAC system, or tear the dash apart and trace the hoses, hoping one of them has a leak, or replace the vacuum switch and solenoid. IIRC, there is an electric switch that opens the vacuum to the vacuum solenoid (for lack of a better term). It seem sas though the leak is downstream from this switch/solenoid valve doober.

PT

ps-I have one to fix on my roommate's LWB, I feel your pain.

 

[lionvt]

Where are the switch and solenoid?

 

[ptschram]

The switch is on the heater control panel. The solenoid valve is on the left side of the heater core which requires complete disassembly of the dashboard.

PT

 

[Craigness]

I personally think your chasing the wrong rabbit.

Any leak from that system should not be enough to cause the type of rough running your talking about.

I thin it might be a multitude of issues amplified by this "Leak".

First off, any RPM based shake like that when vacuum is in question you first check the distributor and vacuum advance for the distributor. It's possible that it's stuck or on its way out. The dizzy timing might be off a hiar and this is being sent over the edge by the additional leak.

The michanical advance might be stuck also.

Did you try plugging off the hose that goes through the firewall?

Plug all the anciallry hoses that have nothing to do with the engine and emissions system. Drive the car for a day or two and take notes.

I bet you still have the problem.

When was the last time you did a PCV hose replacement? Don't forget to use the ACTUAL "T" fitting with the metered oriface.

Soak the flame trap in petrol overnight and then blow it out with shop air.

Check for leaks around the intake manifold. You should do this with an UNLIT handheld propane torch. Open the gas and wave it around like painting all the areas of the plenum and intake areas where you might think a leak would happen. If the RPM's speed up when you get close with the gas, thats your leak.

I have run high milage rover classics wiht tons of leaks but only the real messed up ones run rough at higer than tick over because of a leak in the intake.

BTW, Intake leaks or vacuum leaks tend to go away at higher than tick over rpms.

I would start elsewhere.

Plugs?

Coil - now that would do it.

Dizzy amp module woud do it.

Timing - my brass is on this one.

bad plug wire.

Bad DISTIBUTOR - i'm sure of it.

Post your findings.

 

[Craigness]

one more thing.

Change the manifold temp sensor.

yoiu mentioned that it happens when the outside air vent is switched. It's possible that the motor is getting hot enough to cause an erratic reading on the temp sensor.

It's the one under the fuel rail temp sensor with the exact same plug.

About a 20 spot in most auto shoppes.

 

[lionvt]

These are all good ideas and I thank you for the thoughts. The coil, amplifier, plugs, rotor and cap, and ignition wires have all been replaced in the last 12 months (which I know only says so much on a Land Rover, but still :ack: ). This problem started some months ago so I do not think any of those parts were bad at time of install. I'm pretty confident the roughness is directly related to the climate control switching. The moment I throw the lever, it starts running rough -- it is literally like throwing a switch. However, the rough running is always limited to the 1800-2300 rpm area. If I run it, or idle it, outside of this band width -- no problem. So, I suppose that might suggest timing issues (which is something I don't know a lot about so I can use the ideas).

 

[lionvt]

Is the only hose that goes into the firewall the one from the "Y" fitting where the vacuum line splits to go to the cruise control unit?

 

[ptschram]

Is the only hose that goes into the firewall the one from the "Y" fitting where the vacuum line splits to go to the cruise control unit?

The cruise control is independent of engine vacuum.

There should be a hose running along the wing from a spehrical tank on the inner wing.

 

[lionvt]

I used a fuel clamp to clamp off the firewall vacuum hose, both near the fire wall and near the vacuum pump -- it made the situation better but did not eliminate it. Any more ideas?

 

[rrrandall]

The charcoal purge valve system is ONLY operated in the RPM range that you are having the problem in. I would try putting a dowel on the hose to the purge valve hose and see if that eliminates the problem. You definately do not need the gas tank purge system to operate the engine. It could be that when the purge valve is opened combined with another vacuum leak under the dash(and who knows how many other minor engine problems on a 15 year old car), then that is enough manifold "leak" to screw up the air/fuel ratio. I am nearly 100% certain that the only RPM range that the ECU opens the valve is around 2000rpm.

The alternator is another possibility because it is directly tied to engine RPM. I am not an electrical engineer, so I do not know how an alternator can malfunction only at a specific RPM. Any chance you have a questionable alternator??

One way to determine for sure that it is RPM related would be to drive around in 3 rather than D whilr trying to repeat the hesitation. It will not hurt anything to run the truck in 3. The shifter is made to shift freely back and forth between D and 3 and you do not even have to push the button on the gear lever to change between these gears.

Randall

 

[lionvt]

What do you mean "putting a dowel on the hose to the purge valve hose"? I'm not sure I understand that. But you are right, this is really limited to a very discrete RPM band 1800-2300. When it is really driving me nuts, I do put it in "3" and it runs quite easily at 2500 RPM.

 

[rrrandall]

On the passenger side of the intake plenum there will be 2 or 3 vacuum hoses connected. One of the hoses goes to the purge valve which sucks the fuel vapors out of the canister. The canister and purge valve are on the passenger inner fender. Just disconnect the vac hose and put a 5/16 or 8mm steel dowel(or set screw) in the hose to block it off. When the ECU opens the purge valve at ~2000, nothing will actually happen because you plugged the vacuum hose.

There is a wheel barrel load of devices which are used to purge gas vapors from the tank. Every year they changed the system and nobody understands it completly. You can descreetly disconnect it fron the intake plenum by inserting a dowel in that vac hose.

I don't think that your main problem is the purge system. But I am speculating that when that valve opens it tips the balance enough to cause hesitation. If that makes no difference then you have got to try some other things to fix the problem.

Randall

 

[lionvt]

New observation! Okay, still having the problem. And with the cold weather I recently turned on the heated windshield -- and guess what -- the problem with the intermittent hesitation would come and go as I pushed the heated windshield switch. Now I know that the climate control flap is electrically actuated but vacuum powered -- so is this some type of spooky electrical problem (in an English car?....)?

What could possibly be the connection between these circuits and the engine hesitation?

 

[az_max]

New observation! Okay, still having the problem. And with the cold weather I recently turned on the heated windshield -- and guess what -- the problem with the intermittent hesitation would come and go as I pushed the heated windshield switch. Now I know that the climate control flap is electrically actuated but vacuum powered -- so is this some type of spooky electrical problem (in an English car?....)?

What could possibly be the connection between these circuits and the engine hesitation?

Alternator?

 

[lionvt]

Turning the heated windshield on causes this 100% of the time. I'm guessing something related to the amperage draw of the heater? So, if the alternator is at fault -- how would I diagnose this? There is enough juice the rest of the time. The alternate light does not come on.

 

[az_max]

Turning the heated windshield on causes this 100% of the time. I'm guessing something related to the amperage draw of the heater? So, if the alternator is at fault -- how would I diagnose this? There is enough juice the rest of the time. The alternate light does not come on.

The only time I pushed the windshield heater button, the engine died. So I don't push that button anymore

Autozone or checker can check the alternator while it's on the truck.

 

[landrovered]

Is you tach needle jumping around? That is a good sign that it is on the way out. If it is not then I suggest that you check your ground straps. They often get grimey or corrode. Weird gremlins like this can be traced to grounding. If the primary ground for the dash based climate and defrog is not grounding properly then it could be grounding through any number of other pathways until it uses the amplifier or coil ground. This could disrupt the firing for sure.

 

[lionvt]

It's definitely related to over all amperage draw. For example, if I turn the AC and the head lights, it will happen. Or the heated wind screen and just about anything else. So, is the alternator flaky? As you can see by the dates on this post, its been going on for a while but not getting worse.

 

[loudbay]

Do your lights get brighter as the engine revs? Someone else mentioned the tach needle jumping. I say alternator or grounds. When I had 'fast' efi cars, I almost always added an extra ground strap... or two.