Help me fix this annoying flat spot!

[explorer-aus]

I have a flat spot at about 3,500 rpm to 4,000 rpm. It is especially bad when under heavy load eg. going up a very steep hill and attempting to accelerate. It can be so bad under load that i actually lose power under heavy acceleration and slow down rather than the car changing gears. Under gentle acceleration for the most part the car runs well, runs very well with little load (not towing, empty car).

To try fix this, i've rebuilt my transmission, changed coil pack, leads, plugs, cleaned MAF. But to no avail.

The engine is a 4.0 SOHC, 97.

I'm suspicious of my CATS being blocked up, does this sound like symptoms of this? What are your suggestions, has it been experienced before?

 

[2000StreetRod]

hill climbing vs cats, VIS

Long distance steep hill climbing can be hard on cats if the PCM switches to open loop due to engine load. The mixture runs rich and the cats overheat. Do you have any DTCs indicating poor cat efficiency?

You might try testing the TPS: Ford Explorer - Ranger TPS Test Procedure

Another possibililty is a defective variable induction system (VIS). The PCM should activate the VIS vacuum motor above 3,000 rpm to change the length of the intake runners. The vacuum hose to the motor could be split, the actuating rod may be dislodged from vibration, or the actuating lever may be sticking due to crud in the intake manifold from the EGR.

By the way, congratulations on replacing your camshaft timing components. What modifications were required for you to use the 4USA timing chain kit?

 

[explorer-aus]

Thanks for the reply StreetRod, 80% of my posts get no interest (partially because of timezone differences i suspect).

I'll check all the sensors you've listed and do a code scan (although i've disconnected the battery lately so there will probably be none).

The 4USA kit was great, it only required one modification to fit my 97 SOHC engine. That was on the front of the engine, the guide for the cam chain (non primary) had to be spaced out. Other than that the kit was great. It ended up costing me about $350 to redo rear and front chains including the cost for all the gaskets and seals, without removing the engine. I didn't however change the oil pump chain. Kits been in the car for 30,000kms now.

I also made a little tool to hold the cams in position it was a piece of flat steel with two bolts on either side. The steel ran through the mark on the ends of the cam and the bolts were adjusted to hold it in the correct position.