FRporscheman

To start off, this is a 1993 968 with almost 250k miles.

The car is hard to start after sitting. Some days it takes over a minute of trying. Today I checked the fuel pressure, and it goes down to zero in a matter of hours, so I'm guessing that's the cause of the hard-start, but it might be important information for the stumble problem so I mentioned it.

While driving, if the rpm is low, it will misfire and stumble badly when I step on the throttle. The more throttle I give it, the more it stumbles, and the lower the rpm, the worse the stumble. When it's cold, the threshold is a little higher, so maybe 3500, and when it's warm it's more like 3000.

It's been getting worse gradually. First, it would go away after a few minutes, now it's always there.

The car drives perfectly at high rpm. Whatever the speed, the load, the gear, the throttle amount, it doesn't matter, it runs great and with the same power as before.

Things I've already done:
- checked fuel pressure while running, steady at just below 50psi, a little higher with throttle open.
- new fuel pump check valve
- new fuel filter a few years ago
- swapped in a friend's MAF
- hall sensor terminal was repaired a while back (new dongle spliced on)
- injectors cleaned professionally 8 years / 20k miles ago
- also put on new cap/rotor/plugs/wires 8 years / 20k miles ago
- note that on this car, the CEL blink function does not work. I've never been able to get it to blink. A friend has durametric but the only codes I could pull with it were misfire codes (duh).

My theories:
- it's not the TPS because it runs fine at high rpm, regardless of throttle
- it's not the FPR
- next thing I'll check is the injector spray pattern (and to confirm an injector isn't leaking)



This problem is really driving me nuts. I'd appreciate any ideas or input.

V2Rocket

Just for good measure I'd throw a new fuel filter and spark plugs on it, 8 years is long enough
And 250k miles is quite an achievement!

First guess is some sort of vacuum leak, torn or loose hose somewhere.
At low RPM it would be more pronounced causing problems - high RPM the air quantity and velocity in the manifold is much higher and the leak wouldn't be so noticeable.

Also, vacuum leaks are worse while running cold and not so bad when warm.



....alternatively, when was the last time you serviced the Variocam? Wondering if maybe the solenoid is stuck, putting the cams in the "retard" position which would make it idle normal, but drive sluggish in the lower RPM range, getting better as Revs climb.

mytrplseven

I'd recommend replacing the fuel pump as well as the filter. Most pumps on the market include a new check valve, which is causing your fuel to drain out of the lines faster than it should. While you're in there, check the fuel lines for kinks. Also, pull the fuel pressure regulator vacuum line and see if you can smell gasoline in it. If so, replace the FPR.

FRporscheman

I'll try new plugs first. I don't think it's the fuel filter, because it runs great at high rpm when there is more demand for fuel. I already have a new check valve which went in earlier this year. The pressure must be getting out somewhere else.

I guess it won't hurt to try driving around with the variocam unplugged. But it can't be that - the stumbling is very pronounced, way worse than a few degrees of retarded timing.

mel_t_vin

If the check valve is not the root cause of the fuel leak down, then it would have to be either a breach in a line or a faulty injector(s). Were the injectors rebuilt by a quality shop?

Agree with Spencer...eight years could be a long time for plugs, especially if they've been running in a rich environment. Did you end up using a quality set of wires? Pull the plugs and post a couple photos.

joes

I agree I don't think it's fuel. Could be plugs but I don't think so. Any kind of problems especially if it won't start is to determine if it is "fuel or fire". When you are trying to start it get some help to look for spark or intermittent spark at the coil. My money is on the flywheel pickup dirty or bad connection.

FRporscheman

Fuel injectors were cleaned by one of the well-known places (I forget if it was Witchhunter, Marren, or someone else).

The plug wires are Beru.

I bought a set of NGK plugs, BPR6EY v-power, which is what the guy at the store gave me. I just did some research and found out it is compatible, but the BPR6ES is the specified plug. Anyone have any experience with the v-power version NGk plugs? It looks like it just has v-shaped electrodes... not sure why.

FRporscheman

I went to put in those NGKs and I realized I didn't have my gapping tool. But I had spark plug #1 out already so here's what it looked like.

I forgot to mention before that this engine eats oil. I need to add maybe 2 quarts every 5000 miles, and friends who drive behind me say they see puffs of blue come out of the exhaust. Just in case that's relevant.

951and944S

I wouldn't replace a fuel pump that passes a test for spec just for the heck of it.

Someone above mentioned "fuel or fire" but you also need compression for proper combustion.

What is the compression like....., you never mentioned.....?

The deposits on those plugs look like aluminum from here.

T

FRporscheman

Haven't had time to work on it, I've been busy with my 951.

But here are two new pieces of info:
1. when it's cold/been sitting, the car starts much faster if I open the throttle.
2. while driving, when I let off the throttle completely, it cuts off the fuel harshly and thus bucks forward - and when I touch the throttle again, it harshly resumes fuel delivery and thus bucks backward. It reminds me of an early 944 with the shudder, but 968s shouldn't do this.

V2Rocket

Sounds like a TPS problem maybe?
You ruled out the TPS (because it runs fine at high RPM) but the 968 has a pot type TPS...pots wear out in the range they are most used.

Chances are your car gets most used at very low throttle inputs for cruising...hell, maybe its the original and 250k miles have worn a hole in the pot track


Also, 2 qts in 5000 miles is hardly "eating oil". Porsche's oil-use spec for a healthy 944-series engine (including the 968) is 1L/1000km of "enthusiastic driving".
That's over a quart in 620 miles.

FRporscheman

If that's true, then why does it stumble at any amount of throttle, and even harder the more I open the throttle? I guess I could test the TPS anyway but I really doubt that's it.

I was thinking I might have a leaking injector. So a cylinder fills with gas and can't start until it gets blown out or if I open the throttle and give it some air. Also, it could be running very rich in the same cylinder while running. Just a theory at this point - hopefully this weekend I can mess with it.


Yeah, but that spec only covers the amount of oil that leaks out of the oil pan gasket and lower balance shaft rear o-ring .

CrookedRacer

I know your valvetrain is completely different than mine, but maybe it's not that different after all...

I had severe stumble on acceleration, and I did all the stuff you are doing... rebuilt the intake system and verified each vacuum line and connection, sent the injectors out for cleaning, rebuilt the throttle body, throttle position sensor, cleaned idle adjustment valve, and more.

Then I broke down and had an expert drive it, and he called it.

https://rennlist.com/forums/924-931-...track-toy.html

I really don't know if we share the same valve spring design... my car wasn't using a lot of oil either. Just a thought.

FRporscheman

Thanks for the tip, I'll try to find someone who can drive my car and determine that.

But I drove the car at least 30 miles today on several errands, and it was running fine the whole time. Then I went out with it later in the evening and the stumbling occurred a few times.

I changed the oil today, and didn't have time for anything else. Next is the spark plugs, then swapping a different set of wires.

CrookedRacer

Probably not the same thing then... Mine was not intermittent... It hesitated every time, although less severely when warmed up.