Land Rover Repair: 1997 Discovery I Rough Running When Hot Following Head Gasket Replacement, head gasket repair, 3m scotch brite

Question
John,

Regards.  I run a small independent repair shop specializing in early model Land Rovers (Discoveries & Range Rovers).  I have seen and experienced many of the common problems you have outlined in your writings.   I thank you for being so generous with your mechanical expertise.

I recently replaced the head gaskets and main bearings on a 1997 Discovery I with 135k miles(GEMS engine w/ AEL).  The head gasket repair was performed due to excessive oil leaks and carbon fouling.  The main bearing repair was performed due to a loss of oil pressure at stop lights when the engine was hot.

Following the repair, the vehicle runs perfect until it really warms up.  After idling/driving for 15-20 minutes in hot summer weather, the vehicle stumbles and hesitates with any application of the throttle (loaded or unloaded).  There are no fault codes present and the live data seems to be what it should be.  Once hot, the vehicle acts like there is a clogged converter.  The engine stumbles around 1500 RPM's and will not produce any power in gear.  As I said, the vehicle runs and drives perfect on a cold start.

The heads were reconditioned by my local machinist with whom I have a very good relationship with.  The heads I installed were originally from a 1996 Discovery I.  My machinist pressure checked the heads, re-surfaced them, cleaned the carbon off of the exhaust valves, re-ground all of the valves and cleaned up the seats.  The guides were fine and seals were replaced.

All parts were cleaned using conventional methods and I cleaned the cylinder deck and piston tops with 3M Scotch-Brite.  Everything was rinsed with brakleen prior to re-assembly.

The fuel pump in the vehicle is brand new (Rovers North Proline) as is the filter.  All of the other parts were also from Rovers North (gaskets, head bolts etc.)

What do you think would cause this situation?  Other than gaskets, the only new parts I installed were the spark plugs.  I used Champion RN11YC4 plugs.  The car ran fine prior to the repair (minus the carbon fouling issue).  I have completed this same job on many Rovers and have never had an issue like this.

So far, I have swapped the Mass Air Flow sensor with a good known used unit and there was no change in performance.  The cooling system is completely bled and there are no abnormal sounds, leaks or conditions.

What do you think?

I appreciate your wisdom on the matter.

Thank You,
Steve

Answer
My first thought is that there is an assembly error.  You say it did not do this before, and a valve job should not produce any symptoms like this, so i would look to assembly.  Specifically, I would look to see if you swapped the connector on the coolant temp sensor or any of the other connectors on the front of the motor,   

If all that proves to be correct I would return to the live data which you say is right.  It can't be.   I would drive the truck while it's breaking down and have someone watch the screen on the T4 or whatever system you see live data on.   If the truck is stumbling some readings are going to be way off.  You may have missed that by watching at idle when everything is okay.

Finally, I would make sure the fuel pump is really making the desired pressure.  Poor wiring back there could give voltage losses.

I think one of those three things will reveal the answer