goodspeed928

Whell year is coming to a end up here, so I took here for a hot run.
There's a good spot to run on that I can get 140 with no sweat.
So I put it to it and at 115 or so she FLATED,
I let off, She was running fine, and still is. but dut not like 115 and +++

heinrich

That's not right.

Drewster67

Throttle cables
Air Intake/Air filter

Would be the first things I would check.

John Struthers

Fuel filter.
Easy swap.
Fairly cheap.
Instant results ... if its the fuel filter that is...
I think the flat spot creeps up because:
1. Most of us don't run much over 3200 to 3600 rpm over long stretches of time.
2. While we probably would notice on extended kickdown or winding out heading for the big 3 digit numbers-65% or better throttle we don't do it that often so no mental comparison.
3. When that puppy really gets dirty you'll notice a flat spot or BOG at 60 or 70mph. If you have a 3spd auto - like me - and use it like a manual you will notice it in second at near 3000 rpm.
The actual feel/sound -for those of you who know - is like the fan clutch being locked up on a GM 350 C.I., auto. Seeing as how this is a Porsche 928 it is a scary situation because the mind isn't collating the info and your brain starts to imagine bent valves, frozen lifters, holed pistons clogged injectors (which one?), failing coils, bad wires deteriorating brain, hot wire, bad plugs or plug wires.
You can still drive her but change that filter! Order it now! then go for your end of year flog the
sHARk /adrenal gland run.
Clean the damn air filter, check your fluids, tire pressure, ride height, and tranny fluid level.

IcemanG17

Greg
Do you mean a flat tire or the engine hit the wall and couldn't go any faster? Your car should pull strong into the 150's easy. Mine shifts out of third at about 140! Scared me, but I was smiling.

John Speake

Could be MAF aged = weak mixture = low power. Your car has 139k on it. has the MAF been changed ?

What work has been done to the car since your last good max run, how many miles since then ?

Check out all the other good suggestions...........

Thaddeus

One other thing... if there's some pinging at higher RPM's, the knock sensor could be preventing going higher... or if you have a bad knock sensor..

Just an idea I'm throwing out there, you understand. No idea if it's the true cause.

John Speake

Hi Thaddeus,
Good thought but the car is an '86 early 32v and doesn't have knock sensors............

Shane

figgen

Keep the suggestions coming. I have a similar problem with the '85.

My work to date:
New MAF
New fuel filter
Cut cats off and welded x over
New muffler (ANSI)
Plugs are 1 year old
Caps are 1 year old
Wires are 1 year old
Timing belt 1 year old and tensioned properly
Cam notches line up +/- 1/2 tooth with cam belt cover notch
Air filter < 6 mos old
Throttle cable tightned



My 4th gear syncros are toast so I have to shift from third to fifth, but I can accelerate in fifth up to about 115 then it just holds steady.

The only thing I'm thing might be fuel pump not putting out enough volume...that being said I can rev to red line in 1,2,3 no problem, it just seems to be top end limited.


???

PorKen

Tired ignition amplifier(s)? I had a similar problem with, albeit an early, CD box, which didn't allow me to go over 5000 rpm.

Plugged hose to the the EZK?

AFAIK the only way to be sure about the cam timing is to pull the 32V valve covers and set them per the WSM procedure; there's a two tooth range, independent of the cam gear mark.

mark kibort

you mean shifts out of 4th , right?? 3rd gear max is 100mph in the S2s and 111 in the S4s, while in the GTS and GTs get out of 3rd sub 90mph.

there is no reason to get a flat spot at 115, as we all know, it only takes 120hp or so to break this speed level. so, if you cant get any faster, you have a major issue, and its not knock sensor! (knocksensors cant kill 100hp by backing off timing!) its probably throttle linkage, maf as was offered, or other major issues.
also, check to see if you have normal pulling in 3rd gear. should be 7-9 seconds 60 to 100mph. if you dont want to speed that much, shoot for 3 seconds from 60-80mph in 3rd.

missing? smoke?,

mk

mark kibort

even if your synchros are toast in 4th, you should be able to easily fit the gear in by careful speed matching. try your test then. in 5th gear, the car has the ability to EASILY acellerate from 60mph all the way to top speed with no issues, if there are NO issues. (ha ha) like i said, it only takes about 100hp to get a car like ours to 120mph. i dont know what your exact dyno looks like, but even at lower rpms, the hp of the 928 is not bad in the low range. figure 150-170 hp min. at least even in 5th gear at 3000rpm (or 100mphish).

try and test your car in 4th and see what you get. sounds like you have a major issue as well.

mk

AO

Figgen-
If you can take her to redline in 1 thru 3, but not in 5th, then your car obviously is not producing enough hp to get you over the drag you are experiencing at 115. I'm no expert, but it would seem to me that this would indicate that your A/F mixture is not optimal. My guess is that, if your air filter is not plugged, that somehow your engine is not getting enough fuel.

Now, could it be that the MAF is bad and not measuing air flow correctly thus affecting fuel? (again I'm no expert here - I have an L-jet).

What about a weak fuel pump? Perhaps not enough pressure or flow?

I'd get a fuel pressure guage and stick it on there to see what's happening. If that checks out, I'd see if you can swap out the MAF with a known good one. Hope this helps.

figgen

Fuel pressure gauge was next on my list of things to do. In my opinion my fuel pump is louder than I would like to hear, that being said, it's < 2 years old as well.

No difference with MAF swap performed last summer with a new unit (still in the car)

I agree Mark, I should have no problem in 5th going to red line from third as the RPM are sufficient unfortunately my testing days are over with more snow falling 3 days ago and the forecast....get ready for winter....so unfortunately the search continues in the garage only.