what could cause this?

Johan123m · 07-27-2006, 01:07 PM

The engine of my 2.4T misses at 5000rpm, under load, in every gear. It doesn't backfire or nothing, it just feels like a rev-limiter is installed at 5000rpm!
What could be the cause?
Car has new sparkplugs, sparkwires, rotor, points, new gasfilter, recent valve-adjustment and timing is set...

Regards,
Johan

Early_S_Man · 07-27-2006, 01:33 PM

Johan,

Sounds like a fuel delivery problem ...

Test the fuel pump output into a clear glass container of at least 1.5 liter capacity. An MFI pump should put out a minimum of 900 - 1000 cc in one minute. A CIS pump should put out at least 750 cc in 30 seconds.

Look for rust and debris in the fuel ... if you observe any, proceed to inspecting the hard fuel line in the tunnel. Blow out the line with compressed air and repeat the timed volume test.

BeauBlues · BeauBlues's Garage

Did you change the distributer cap? When my cap and rotor gets worn out, it does the exact same thing.

kepperly · kepperly's Garage

THOSE YEARS HAD SEVERAL DIFFERENT DISTRIBUTOR ROTORS AND CAPS. ARE YOU SURE THEY ARE COMPATIBLE.

KEITH EPPERLY
87 SLANT NOSE TURBO LOOK CARRERA CABRIOLET

Johan123m · 07-27-2006, 01:55 PM

Yip,

Changed the cap too.
Gonna check if it's the right cap&rotor and try the pump-testing-method described by the early S-man
I'll let you guys know if it worked.
Thanks
Johan

kepperly · kepperly's Garage

good luck my friend, sometimes these cars are truly a *****.

keith epperly
87 slant nose turbo look carrera cabriolet

kach22i · kach22i's Garage

I just read that if the spark plug gap is too great not only can it stress your coil and other electrical parts, but the spark may prematurely ground at the base of the spark plug and not at the tip where you want it. This will cause missing at higher rpms.

Too small of a gap may cause running rich at idle, leaning out at higher rpm's.

Did you also recently change your plugs?

Johan123m · 07-27-2006, 10:08 PM

Yes,

I just changed the plugs. They had a standard gap of 0.8mm instead of the 0.5mm that is specified, but I asked the question here on the bbs and everyone told me that with those plugs 0.8mm would be ok.
I used bosch WR5 DC+ instead of bosch W3 DPO on my 2.4T

?
Johan

kach22i · kach22i's Garage

Quote:

Originally posted by Johan123m
Yes,

I just changed the plugs. They had a standard gap of 0.8mm instead of the 0.5mm that is specified, but I asked the question here on the bbs and everyone told me that with those plugs 0.8mm would be ok.
I used bosch WR5 DC+ instead of bosch W3 DPO on my 2.4T

?
Johan

I'm not enough of a plug guy to know if they are okay. Typically if enough people agree you are on the right track - that is until a real expert chimes in.

kach22i · kach22i's Garage

I just changed my stock plug gap from .028 inches to .035 inches. This change fixed the running rich at idle problem, the little back fires when I let off the gas, and the engine in general just runs smoother.

I should clarify that my spark plug gage has .025 and a .03 inch wire loop. What I did the time before last was make the gap a geneorus no touching .025, yet still to small to fit the .03 inch wire in. I thought this was ending up at .028 inches, perhaps it was not.

I think there are other issues with my car which were causing the buffeting under load at low speed/low rpm, running rich, and backfiring (such as the WUR). However increasing the gap slightly has the car running better than it ever has, including starting cold and starting when warm.

Warning: if you regap your plugs larger than what I did you may burn out your coil, burn out points if you have them (I have a Pertronix), cause your spark plug wires to arc, plugs to arc and ground out at hight rpm, and a host of other issues. I chose .035 inches after reading more than a dozen threads on the topic, I think I have the gap right on the edge of trouble, but not in trouble, time will tell. Until that time, I have a well running car that I'm still sorting out and seeking to return to factory spec in most regards.

Johan123m · 07-30-2006, 06:01 AM

Hey Guys,

It was the rotor! Changed the rotor and it pulls strong!
No hestitations whatsoever!
Thanks!
Johan

kepperly · kepperly's Garage

Johan123m,
The simplest things sometimes are the hardest to find, congrats on another well running Porsche for someone else to drool over.

The older ones had significant while in production changes and always consult Pelican BBS,we and Pelican are here to help.

Keith Epperly
87 slant nose turbo look carrera cabriolet

kach22i · kach22i's Garage

Your distributor could be wearing out, causing more wear on the rotor.

One guy in the forum once said the distriubtor should be rebuilt every five years, as it's that important to a good running engine.

One trick I learned is scraping the contact edge with my keys to make it shinnny (in a jamb). I did that once and it turned out to be the points, so I installed a Pertronix system.

Johan123m · 08-01-2006, 06:57 AM

Hey Kach,

I'm just replying on your post, so I haven't used the 'search' yet, but rebuilding a distributor, is it a DIY-job?

Johan

kach22i · kach22i's Garage

Quote:

Originally posted by Johan123m
Hey Kach,

I'm just replying on your post, so I haven't used the 'search' yet, but rebuilding a distributor, is it a DIY-job?

Johan

I've seen the cut-away parts drawings of a distributor, no way would I even try it myself. There are a few on this board that have no fear - more power to them.