Flat spot betweenn 5600 rmp and 6000

#1

another mechanical question…

During the Sebring 1 1/2 hr enduro on friday I ran into a engine issue. When I would hit 5600 rmp in 4th gear the motor would slow to a crawl in terms of rev’s between 5600 and 6000, than once I got past 6000 it would move at the anticipated rate to the rev limit. So basically, the motor was totally flat between this rmp range. It was so bad I could rarely get the car past the 6000 rpm level, when on the previous day I would have to feather the throttle to avoid the rev limit. Any idea on what might be going on?

#2

When I had those symptoms it ended being slightly frayed plug wires within the loom. They were all arching to one at certain RPM’s.

Others have had the vibration dampener loosen on the front of the motor and had the same problem.

It could be a number of things. I recommend the approach; throw parts/dollars at it until its fixed. :side:

#3

Carlton, I seem to recall Robert Patton fighting this problem through several sessions last year. I don’t remember him mentioning plug wires as the problem/solution, suggest contacting him.

#4

Patton’s was the vibration damper/harmonic balancer (the notched wheel on the front of the motor)issue I mentioned above. His was loose.

#5

What’s your fuel pressure look like at speed? A friend recently had an upper-rev-range hesitation due to a trashed fuel filter.

#6

JP nailed it… Throw dollars at it until it’s fixed. Start with the balancer (make sure it’s bolted down correctly and tight), then the CPS and the AFM. Then make up a theory in your head, replace parts, test. Or take it to a shop where someone can fix it who can actually diagnose the problem (check for correct spark/fuel at that RPM). That would be my firing order.

#7

Vibration dampner problem shows up as a bad cutout/misfire.

My thought: if it feels like a strangle-hold at the higher rpm you should change the air flow meter as likely the 20 year old brushes in the wipe arm are worn out/not making contact.

Good luck chasing engine gremlins.Your best bet to find the problem is to borrow “known good” parts from a friend in the pit area during a practise session.

Regards, Robert Patton

#8

Okay, Carlton, what was the problem?

We can all add it to our list of things to check.

Regards, Robert Patton

#9

Patton wrote:

[quote]Vibration dampner problem shows up as a bad cutout/misfire.

My thought: if it feels like a strangle-hold at the higher rpm you should change the air flow meter as likely the 20 year old brushes in the wipe arm are worn out/not making contact.

Good luck chasing engine gremlins.Your best bet to find the problem is to borrow “known good” parts from a friend in the pit area during a practise session.[/quote]

It isn’t difficult to test the AFM to determine if it is the cause, or to repair it if the resistance track is worn. See http://e28-535i.com/technical/AFM%20Repair.pdf