The V8 Owners Forum -

ok a wierd problem im having with my 3.9 v8 that it runns great at low revs but asoon as the revs get to about 3000rpm it starts to hesitate and the revs jump abit like its on the rev limiter but in a bad way and it wont pull anymore past 3000rpm

anybody got any ideas wot this could be??????

oh my engine has loads of mods

ported heads
higher comp steel gaskets
4.0/4.6 airflow meter
crower cam shaft
adjustable push rods
adjustable FSI fuel reg
blaster 2 coil
rhoads lifters
KnN air filter

coil breaking down?
Is it distributor ignition of all electronic just duff condensor used to give these symptoms on a Mini?
Haul effect cam sensor gone "funny"?
fuel starvation due to blocked line? crap in filter?
Petrol cap breather partially blocked?
Wrong preload on the lifters allowing them to pump up and hold the valves open?
Fuel pump not up to all the mods (though I would expect this to effect it at higher revs).
run out of ideas
Mike

is this a freshly modified engine??
if it is, I would be looking electrical, if it was leaning out you would hear it popping back thru the manifold,
Check each manifold exit, and see if its colder than the others,
have you checked the firing order, its so easy to get it wrong,
before you restart the engine, take out all the plugs, hold the throttle wide open and crank the engine over, clean or replace the plugs and Gap them to 0.032", I had big trouble with plug gaps being too big and real misfire problems,
and as kiwicar suggests Lifter preload

I seem to recall that your compression tester was showing 15 bar when you built this engine.

Not trying to be "funny" but maybe its detonating?

Pete

sidecar wrote:I seem to recall that your compression tester was showing 15 bar when you built this engine.

Not trying to be "funny" but maybe its detonating?

Pete

220psi, is not too bad, Think it works out to abour 10.5Cr dynamic, But you could be easily right about it pinking big time, but wouldn't the temp gauge be off the scale??
if thats about right, I deffo would be running on Optimax,

ian.stewart wrote:

sidecar wrote:I seem to recall that your compression tester was showing 15 bar when you built this engine.

Not trying to be "funny" but maybe its detonating?

Pete

220psi, is not too bad, Think it works out to abour 10.5Cr dynamic, But you could be easily right about it pinking big time, but wouldn't the temp gauge be off the scale??
if thats about right, I deffo would be running on Optimax,

Not sure if the temp would go up or not, I guess if you let the motor detonate for 3 or 4 minutes that might be long enough to get the extra heat into the coolant system so that it shows up on the temp gauge.
(Assuming that the motor will hold together that long!)

I suspect that at low RPM the VE isn't good enough the allow the pressure to go too high. (No ram effect, and blow back etc, also not all of the advance has kicked in below 3K RPM).

I guess the dynamic compression will be effected by loads of factors which is why (rightly or wrongly) I just stick with the static figure when working things out. I think that it will work out at something daft like 15:1

(My research on the web and in books shows that 180 PSI is a good max PSI figure to aim for).

If the CR is round 15:1 then optimax ain't going to help, not unless you are talking about the diesel stuff!

I could be wrong about all of this and I hope that I am for the sake of DiscoV8's motor!

Like I said, I hope its something else.

Pete

I'd try a different fuel. I had similar issues years ago with a 3.5 that wouldn't run past 3000rpm on unleaded but was fine on 4-star.

Finally tracked it down to the wrong coil.

Chris.