GerritD

Hi guys & girls,

since quite some time I encounter bad acceleration, even hesitation of the engine especially during heavy acceleration (pedal to the floor but only under 3000 rpm) ,
especially when the car has not run for couple of weeks.
Car is a Porsche 928S Euro 4.7L 300HP with Kjet CIS
When I have driven the car for about 30 minutes, the hesitation is much less than with cold engine...so temperature of the engine has an influence but hesitation is still there.
When I accelerate till 5000 rpm, there is no hesitation in the higher rpms (above 3000 rpm)
Also when accelerating normally (slowly acceleration) there is no hesitation at all...

Here some things I already replaced in the meantime :
- new fuel pump
- 8 new fuel injectors
- new WUR
- new fuel accumulator
- new fuel filter
- new intank filter

Fuel pressure cold (@10°C / 50°F) = 1.1 bar / 15 PSI
Fuel pressure warm engine = 3.5 bar / 50 PSI
Fuel rest pressure (leak test) : after 10 min = 2.9 bar / 42 PSI
after 20 min = 2.8 bar / 40 PSI
after 30 min = 2.7 bar / 39 PSI
Everything seems to be normal.

Is there anyone who can point me in the right direction where to look for ?

I heavily suspect the Fuel Distributor as it has never been overhauled... Could this be the culprit ?
Or do I need to suspect the Kjet air meter plate ?

thanks in advance

jpitman2

Has the mixture been properly adjusted since all the above work was done ? What is the system pressure behind the above control pressures? Have you done a delivery rate test ?Sounds like its going lean to me. FWIW, I have a system of 75psi, and a thoroughly hot control of 46psi and have no issues with driving.
jp 83 Euro S AT 57k

GerritD

Colortune was OK (blue). Color of my sparkplugs was light greyish (oh yeah, I also replaced my sparkplugs by the original Bosch ).
System pressure is 84 PSI.
Delivery rate, not sure. Should this be about 1350cc /30 secs ? Measured at the return line of my fuel distributor ?

jpitman2

Thats the delivery rate you should see. The Colourtune will be hard to see under load and throttle. To verify going lean, can you try to get an Innovate wide band sensor up the exhaust pipe, and have a passenger watch the mixture readings while you reproduce the symptoms. If that doesnt tell you anything, I would start looking at electrics - I have seen reluctor type triggers have strange failures. How is your coil, and all plug leads are good - put a meter across the ends of each lead, and twirl the wire to flex it, which can expose internal failures.
jp 83 Euro S AT 57k

Mrmerlin

Replace the crank position sensor with a Bosch part.
verify that the distributor rotors are both pointing to the respective hash marks with the engine at TDC

The Forgotten On

It's an old K jet with a single dizzy. No CPS to worry about here!

GerritD

interesting fact : when engine idles at stationery and I move the throttle by hand to revv it up, it does not hesitate.
I have no CPS .
Coil is also renewed and is a Bosch. Plug leads are not renewed but resistance is all OK. But I will check them again by twisting them to
see if there are any internal failures.
Is there a way to check the plug leads with engine running?

jpitman2

Not that I know of unless there is a misfire you can produce while stationery - then pull or short leads until the engine note does NOT change, and thats your problem cylinder. Check your dizzy cap for tracking ! I once pulled a head (rover v8) to fix a burnt valve and misfire was still there - spark was tracking inside dizzy cap. If there is dust or moisture in the cap you can get tracking under load.
jp 83 Euro S AT 57k

gbgastowers

I guess it’s the fuel distributor. Also I would install a permanent fuel/air mixture gauge. Innovate makes a good one. I had to get a bung welded into down exhaust from engine and installed the gauge on the right side corner of passenger under dash tray. It helps tremendously knowing what your mixture number actually is.

Mrmerlin

Sorry for some reason i thought we were working on a 4.7 2.2 MAF ,
Please post a picture of your engine,
whats the condition of the green wire.
and the coil wire and plug leads?

marine928

GerritD,
I recently experienced very similar issues ( see my post, vibration under acceleration ) I have an 87 that until recently, has been virtually problem free for the past 10 years. I spent a lot of time and money chasing the two main issues which included hesitation and erratic idle, especially after it warmed up. Nothing major worked. My mechanic returned from the Philippines for a week, drove my car and told me that my low rpm vibration was due to a misfire or bad injector. He did a simple test and discovered a bad #8 injector. Long story short, replaced the injector, shudder went away. 1st problem solved after thinking it had to be something major and doing unnecessary and expensive repairs ...
Problem number two arose after fixing number 1. Prior to vibration issue, my car was running perfectly. While addressing the shudder I had changed all the injectors, plugs, checked the vacuum lines and cleaned the MAF. When everything was put together I encountered the poor idle, hesitation and proceeded to address it assuming it must be something major..ISV, MAF, LF, fuel regulators etc.
Nothing worked and I was getting frustrated because up to now, my 928 had been flawless. I was reading through some old post and saw where someone had had a similar issue and he corrected the problem by replacing the negative battery ground strap. I had looked at the ground strap when I removed it prior to correcting the first issue, and it looked fine for a 30 plus year old part. I went back and this time cut away some of the plastic covering and noticed about a half inch of discoloration. The rest of the copper weave looked perfect but I decided to use the backup strap that came with the car from the previous owner just to eliminate this possibility. I gave it a one percent chance of correcting the problem.
Well like "quid pro Joe" said "I'll be a son of a bitch" it worked. I learned a valuable lesson that we all need to remember especially when dealing with the 928. If you have already addressed major issues and do perform the routine preventative maintenance required or suggested by the knowledgeable and extremely informed contributors of this forum DO NOT ASSUME that any issues must be a MAJOR issue.
The 928 got a bad rap after the 2nd or 3rd owner deferred required routine maintenance which invariably cascaded into major issues which gave the mistaken public perception that these cars are un-reliable money pits. They are not..If you do like many of us and find a 928 that was well sorted and looked after by the previous owner, BUY IT. Then do what I did and before you start driving, upgrade it with all the cool stuff made available by other owners whose love for the car, its history and engineering perfection, compelled them to unselfishly develop and/or research components to make it even better. TT, clamps, fuel lines, alternators, WP, LH, transmission lines, tensioners etc..
I spent over 1500 hundred dollars chasing down what was really a 200 dollar problem because I mistakenly thought that it was my time have one of the major problems you always read about in this forum. Keep it simple..lesson learned.

GerritD

Hi MrMerlin,
here some pics of my engine:

green wire well protected by black tube

engine with custom made fuel pressure gauge


distributor cap is also recent, also internal components

fuel plugs are still in good shape



new WUR


recent coil (1 year old )

GerritD

THanks for your feedback. Very interesting.
I also replaced my 8 injectors by new ones because engine was not idling nicely and also hard to start because of leaking injectors. This is now solved
I will certainly check the battery strap...I am just wondering how this would impact my hesitated acceleration because I have no electronic components to control my injectors,...so current/voltage is not really impacted
The only electronic part is the Hall CPU to provide the spark (via green wire) to my spark plugs.
What I really find strange is that it only hesitates between 2500 - 3000 rpm when I push the throttle really sudden and hard, and once it doesn't hesitate anymore and rpms are above 3000, the problem has also disappeared...
And when I slowly accelerate letting the car come to speed slowly and steady, the problem does not occur...

Shark2626

Are you using an added octane booster in your gasoline?

Not that I’m suggesting it, as I’ve played with it before in my 911 and had some odd things happen under very rapid acceleration.

GerritD

No I am using the standard fuel (Eurosuper 98 E5).
Sometimes, but rarely, I use the Shell V-Power Gasoline. Apparently it is the better gasoline, especially for older cars.