Recently found a 97 discovery SD with the 4.0 at a price I couldn't pass up with the understanding it needed some work. It starts and idles find and has consistent spark but when you increase the throttle the spark will stop until it dies back down and then if you release the throttle it regains spark. I'm completely perplexed. Tried replacing the coil packs tonight with no difference. It had new plugs and wires put on before I bought it by previous owner attempting to find the issue. The only remaining idea I can come up with is damaged ecu but I'm not sure if it influences the spark or just the fuel and air ratio or possibly a short but I have no idea where to start looking. Any suggestions greatly appreciated.

 

One of the sensors, the Throttle Position Sensor, cuts fuel at 1750 when faulty, throws a fault code too..... Check Engine Light on? No? Bulb is burned out most likely. Grab an OBDII scanner and hook up. Don't replace parts until you know what needs replacement, you'll go broke guessing. These are not very difficult to work on but having the Service Manual called the Rave and a scanner will take most of the frustration out of it. The ECU manages the entire motor with inputs from the sensors. ECU's have been known to go bad but it's not common, usually just a faulty sensor or sensors. I have a couple of trusted vendors listed in my signature.

 

Have you checked fuel pressure. Had a 96 doing that and was fuel pump.

 

The problem is electrical in nature. When the rpms increase I'm losing spark as can be seen with a spark gap light. I was planning to change out the coil packs anyways so no unnecessary parts.

 

When the spark cuts out you can smell the exhaust getting richer and if you hold the throttle after it loses spark it will eventually backfire so I am fairly confident the cylinders are getting fuel. Does the ignition have a ICM as well as coil packs? I ordered a manual but apparently it was an ebook so ordering a real copy this weekend.

 

Nope, all ignition fire is controlled by the ECU. Many times when someone has a funky ignition issue it's related to the Crankshaft Position Sensor. Drivers side, down low on the block web casting for the trans. Check the wires to the Crankshaft Position Sensor, they get baked by the exhaust and the insulation cracks/falls off. Check it's connector and see if the sensor is firmly mounted too.

 

I'm out of town visiting family for the weekend, but while the girls were shopping I did some googling and I was planning to check that when I got home. I'd be extremely happy if this was a sensor and not the ecu because I have zero experience in that area.

 

The ECU is easily swapped out, doesn't require anything beyond the physical change out. In some cases it requires the accompanying alarm box known as the 10AS. Some ECU's and 10AS are paired, some are not and it's a crap shoot to know which is and which isn't as seen by several folks who've swapped their ECU's in the past. 97 has a mid year change in the fuel evap loss system so there are two ECU's but it generally doesn't matter what year as long as it's newer than your model year. A pressure sensor was added to the fuel pump in late 97 and the vapor can was laid down sideways with a different purge routine than prior years, not a big deal. The later 97's pressure sensor is also tied into the Throttle Position Sensor which has made some wire troubleshooting tough. The mid 97 change lasted through the rest of the production run that ended mid 99 when the Discovery II was introduced. The two used part vendors listed in my sig are the go to guys for used parts or you can try a wrecking yard. I've bought from Paul and Will for many hard to find parts and generally the parts are as good as new, I trust them both.

 

Got home this evening and after I took the little lady to watch the 49ers play on the big screen, i got home and I'm fairly sure you were right. The wires going to the cam sensor were toasted. I ordered a new sensor since it's such a pain in the *** to get on and off just to be safe. The wires on the harness have been spliced before so was planning to replace them after covering each wire in heat shrink to avoid any shorts. I noticed the outer jacket looked like it was also woven wire, is this a ground or is it just to shield the wires? The previous owner didn't splice it back together. Would it be a better idea to replace the entire harness and if so where would be a good place to try and find one? Thanks again for the tips.

 

Your welcome on the tips. Braided wire is shielding which is grounded.... The signal is in Milli-amps back to the ECU, it requires electromagnetic protection. There isn't any breaks/connectors of those wires back to the ECU, it's direct wired so getting a new harness to the crank sensor will still require a splice job. Paul or Will would be my source for used but good splice in wires. If you really want to go all out there is heat resistant sleeve material you can buy at Summit, JEGS as well as Home Depot to protect your time and investment and keep er singing for a long time to come.

Your providing the footwork and shotgun, I'm just pointing where the birds are.