3.5 V8 EFI - Lack of Power

Hi guys

Developed a problem yesterday with my 1992 3.5 V8 EFI Discovery. I was on the road when I suddenly experienced a decrease in power and the entire vehicle was vibrating.

I switched off and then when I switched back on it started but it did not rev up to 2000+ RPM as it usually does before it gradually drops to base idle speed. It just started and went straight to base idle speed. There is no knocking in the engine but it is vibrating a bit and slight misfiring sound is coming from the back of the tailpipe.

If I step on the throttle while on park it revs. If I put it in drive, it lacks power.

This vehicle has a new coil, new HT leads, new spark plugs and rove fine before.

I also disconnected the stepper motor and it did not rev up as I thought it should when this is disconnected.

Since it starts, I have ruled out the fuel pump.

I'm hoping that one of you great guys in here can give some advice on what to check next!

Thanks in advance.
Ed

edwardbahaw said:

Hi guys

Developed a problem yesterday with my 1992 3.5 V8 EFI Discovery. I was on the road when I suddenly experienced a decrease in power and the entire vehicle was vibrating.

I switched off and then when I switched back on it started but it did not rev up to 2000+ RPM as it usually does before it gradually drops to base idle speed. It just started and went straight to base idle speed. There is no knocking in the engine but it is vibrating a bit and slight misfiring sound is coming from the back of the tailpipe.

If I step on the throttle while on park it revs. If I put it in drive, it lacks power.

This vehicle has a new coil, new HT leads, new spark plugs and rove fine before.

I also disconnected the stepper motor and it did not rev up as I thought it should when this is disconnected.

Since it starts, I have ruled out the fuel pump.

I'm hoping that one of you great guys in here can give some advice on what to check next!

Thanks in advance.
Ed

Click to expand...

I think I would check air flow meter first and then throttle potentiometer. I note you have a changed a couple of ignition parts but not the rotor arm and distributor cap - take the cap off and check the condition inside, look for evidence of track marks and also pitting/corrosion of the pick ups; carefully look at the rotor arm check for cracks and any evidence of arcing.

You say you disconnected the stepper motor - this would not be likely to make any difference because the cone would have been in the correct position to give a stable idle at the correct speed. If you remove the stepper and then start the engine then it will rev at about 2000rpm. The symptoms you describe would not be affected by the stepper motor; an injection problem or an ignition issue could cause power loss and vibration (misfire, most likely) the difficulty is isolating where the problem is because an injection stumble can feel the same as an ignition misfire.

It might also be worthwhile trawling this forum, I'm sure a search will give you a direction to try, there is a huge amount of information about how to deal with starting/running/stalling issues on the hotwire V8 engine.

If you want to try and diagnose this yourself there are two ways to go: 1 - go to a dealer or good indie and get it plugged in, see what if any fault codes have been logged.

2 - Test components with a multimeter and trouble shoot by substitution; this can be time consuming and expensive.