Here goes, 5 year mystery performance problem and need help! We are original owners and love this truck, so we have kept it and relegated it to local town driving, avoiding highway driving. But we have a new driver soon and need to get this old girl running right.

Symptoms:
After car warms up, especially on highway driving Rover begins to hesitate and shutter. This is most typical when the throttle is very steady or very slight acceleration (like on a mild incline). This is a 5 speed, so one of the signs that is misfiring is the gear shift begins to shake. At it's worst when misfiring, if I add steady throttle, it just gets worse. However if I back off the throttle for a moment, and then increase throttle slowly, it will recover. This is most typical at speeds of 55 - 65mph in 5th gear. And a steady throttle can definitely bring it on. If however, I vary the throttle like a wave - I can typically avoid the problem. Seems to run better when truck is at 75mph and higher RPM.

The problem is not limited to highway driving, but will occasionally hesitate in any gear around town AFTER it has warmed up significantly.

Misfire codes for Cylinders 4 and 8, and a multiple cylinder misfire, along with Check Engine light will sometimes come on.

The most consistent symptom is a -25% short term fuel trim in Bank 2 (see Scan info below)

Some History:
We first noticed this problem in 2007 and brought to the Land Rover dealer. They diagnosed as sticking valves, and wanted $3700 to fix, so we put it on hold for a bit. Eventually I ended up removing the heads myself and having them rebuilt by a machine shop in Massachusetts that Robison recommended. Three separate shops have tried to fix, but since the problem is somewhat intermittent and requires highway driving it has been tricky to diagnose.

What we have done: Note that all of these have improved performance but have NOT corrected the main problem.
- Removed and remachined heads and valves. There was carbon build up in the heads.
- Replaced all gaskets when heads were rebuilt - head gaskets, exhaust, intake manifold.
- Replaced all fuel injectors with new
- Replaced Fuel Pressure Regulator
- Replaced Plugs and wires
- Replaced Passenger Side forward O2 sensor
- Replaced Vehicle Speed Sensor
- Replaced Fuel Pump-Found low/degrading fuel pressure (as low as 15-25) as the car warmed up
- Replaced Fuel Filter
- Attempted to replace MAF with All4x4 aftermarket sensor, but truck wouldn't run. Instead, I disconnected the MAF to test. The truck struggled more especially when accelerating from full stop, but the exact same symptoms occurred on the highway with MAF disconnected. I am back to the original MAF.
- Replaced all vacuum hoses on the intake manifold
- Replaced Intake Air Temp
- Swapped out the Throttle Position Switch (but that test was inconlusive). Back to original TPS
- Have not replaced ignition coils, since the misfires were in 4 and 8 and I believe these are on different coils (not that I would rule out anything at this point)

I connected a PLXDevices Kiwi with iPhone Dash Command.
Here are results of the scan log at 6 different stages, all within a single minute, starting from truck running ok, degrading, hesitating, into Open loop, then ok again closed loop.

Running OK


Running OK


Degrading


*** Misfiring/Hesitating*** Notice the -25% Fuel Trim on the Bank 2 O2 sensor and Trim



Open Loop


Back to Closed Loop running OK




Thanks for your help solving this mystery!

 

Bad VSS, this is the "A" typical VSS failure symptoms.
Vehicle Speed Sensor.
It is what sends the road speed signal to the ECU, if it sends a signal higher than the speed limiter (110mph) the ECU cuts spark to the cylinders one at a time to slow you down.
When it starts to go bad it sends a false signal and the ECU will cut spark, you will get mis fires under acceleration, like climbing a hill, taking off from a stop light, trying to pass on the expressway, etc.
If it goes long enough it will set off the check engine light and you will get multiple cylinder misfire codes.

 

Spike555 thanks for your response. I thought the VSS was responsible as well, but after replacing it with a new Land Rover original sensor, the same symptoms persisted. There is one picture of the scan data above that shows the VSS speed as 96km/hour (I accidentally cut the VSS off in the other scans). Looking back now through the Scan frames for the full minute around the problem occurring, I am not seeing any significant fluctuation in the VSS. Am I missing something?

 

Try unplug VSS and operate truck. Of course speedometer will be zip. The VSS does not connect directly to the ECU, instead it goes to a small circuit board inside the dash cluster, and is sent to the ECU, cruise, and service reminder module. So that circuit board could cause the effect as well, and perhaps even the other modules (unplug cruise and service reminder) or wiring harness. If you have a Gulf States vehicle, the VSS is also fed to the MFU to "beep" when you go faster than a flying carpet.

The circuit board pix and schematic are attached. The same board also does the tach I think. Sources would include boneyard and dismantlers.

 

I did not know you replaced the VSS already, my mistake, I missed it.
It could be a cracked circuit board like Buzz said.
Unplug the VSS and see what happens.
What shape is the clutch in?

 

Thanks for both replies guys. I will try unplugging the VSS. Will that throw the ecu into Open Loop and hide the problem?

I'll let you know what I find.

Doug

 

Spike555 - Clutch and slave cylinder new within 2K miles

 

No unplugging the ECU will not throw the ECU into open loop.

 

Spike555 and Savannah Buzz -Holy crap you guys are on the right track! I unplugged the VSS, went for 50 mile ride on the highway and the truck ran perfectly. The Short term fuel trim were in sync (Bank 1 and 2) and never went negative. The Fuel system was Closed loop the whole time. Since i changed the VSS with an OEM unit 3 years ago, the problem must lie between the sensor and the ECU. I will look for a replacement circuit board, and clean the contacts on the connector and see how it does. 5 years...and I am finally just learning about the circuit board between the sensor and the ECU. Thanks so much, I will keep you guys informed how the next few steps go

 

MAF can be cleaned when cold with made for it cleaner, would be best if it is plugged in. just a side note. While it might be the board, it could also be wiring (skinned and touching metal, etc.), or even one of the other two modules "breaking down" that are served by the same output. Before tearing into dash cluster I'd try unplugging those other two modules.