purchased a well-used 1999 Discovery I. Body is in good shape, interior is fine, but the engine.... runs rough and blows black clouds.... There were lots of fault codes when I bought it. Reset the computer and none have shown up since, but it still doesn't run right, though it does drive.

I replaced the fuel filter, then the fuel pump went out, replaced that.
Pulled plugs and wires, checked compression, each registered in 150-175 bracket. Installed new plugs and wires.

My code reader shows Throttle Position Sensor at 94% at idle, does not change with throttle position. Also reports MAF at 0.02lb/min no matter throttle position. If I disconnect MAF, no change to idle or ability to rev engine (stationary). If I disconnect TPS, fault mode induced, RPM increases. Also reports coolant temp at -40F... if these readings are right, I don't see how the engine runs at all....

I removed the TPS, cleaned contacts, tested it with an ohm meter, re-installed. Cleaned the Throttle Body as best I could without removing from the car.

Any suggestions on what to check next?

 

Sounds like a coolant temp sensor might be in your future as well. If it's reporting to the ecm that the engine is -40 degrees it would certainly richen the mixture

 

There's something obviously wrong here. Unplug the fuel pump pressure sensor connector on the fuel pump housing. It's stupid, but they're connected. See if the TPS works. If not, replace TPS.

 

So, I pulled the temp sensor and tested it. It works fine (resistance decreases with increased temp). Put it back in, and the circuit responds to changes in temperature as the engine warms up (as measured with voltmeter). However, my code box still reports -40F (for this and the intake air) even though the dash temp gauge hits about mid range.

Also, misquoted the MAF output. It reads 0.2 with engine off, at idle, or throttle open.

My wiring diagrams suggest junction HJ7 is a common reference for many of the malfunctioning sensors. Anyone out there know where it is? The picture in the wiring manual is difficult to compare to the real deal.

Thanks for your help.

 

Might try spraying the MAF with CRC MAF spray cleaner.

If it's high mileage, probably would be prudent to replace the temp sensor anyway and TPS. Just a matter of time before they go.

Did you by chance put a vacuum gauge on it to see what vacuum it's pulling?

 

Can you post what codes your receiving.

There are two coolant temperature sensors, one for the gauge and one for the engine ECU. The one that Army referred to is the one for the ECU. When it fails commonly the mixture setting will remain heavy. It's sole function is to warm a cold engine to operating temp. So when it fails it will not tell the ECU the engine is warm.

The Idle Air Control solenoid is another source for head scratching. It's just below the throttle position sensor. It's port seat can become thick with carbon. Always a good spot to start trouble shooting a crappy running engine. Remove the carbon from the seat and the IAC's pintle. It helps.

The MAF probably needs a shot of cleaner.

Erik posted about the tanks pressure sensor tied to the TPS, it's worth following his suggestion.

And yes, HJ7 is a good clue to follow up on. I would assume it's in the engine bay harness but if not it's under the glove box. Did I tell you how much I hate tracing Rover circuit splices? I don't know exactly where it is......

As far as the MAF readings go the ECU has it's own unique algorithm. Just ignore the readings altogether. It hasn't failed, the motor would be hard down if it had.

The air temp is tough one. I don't know if it's the air cleaner box sensor or another sensor that's misreporting temp to your scanner. You can check the air box one by placing it in ice water and then a flame to check resistance.

 

HJ7

 

I haven't heard this one. What is the HJ7 in the picture, and K140? What are they?

 

I wouldn't worry about the MAF... our trucks report rover-ese and the values reported are in a different category than what is actually going through. Save it for last i guess, fix the other issues first, then see if you think you still need a MAF

 

So, I pulled the Idle Air Control solenoid. Looked very dirty, so I cleaned it, ran a brush into the hole, put it all back, now the truck refuses to start. Currently applying a battery charger... The battery that came in this seems to not work very well. Is it normal for the fuel low light to come on when cranking, no matter how high the fuel level is?