Heirsh

background:

1990 C2 coupe, 94000 miles

recent work: replaced DME relay (old wasnt bad just precautionary as it was an older type), replaced coils (secondary was bad)

work history: Things have always been taken care of by previous owners at a dealership. Dist vent was done (sorta, the dist is getting air at least). The connector work for the air flow sensor was done (I verified it myself). It is not overfilled with oil, nor can I find any indication of it ever being overfilled. Recent spark plug replacements. Wires look in great shape. I did not look in the rotor caps.

Car runs fine in cold or warm. Starts fine in cold or warm. Idle is not perfect, but pretty darn close. The injection system seems to be in good shape now after fixing my general power and hesitation problem with 2 new coils. Love it when things are easy.

problem:

I have an issue with power loss when I am going slightly downhill for 20 seconds or more. If I am going more than just a bit downhill the problem does not occur. I'd love some advice on what to look for. The problem doesnt seem to care what temperature it is outside. When the problem happens if you let the car idle it is seemingly fine, but when you apply gas it runs worse. No amount of gas pedal will help until it clears itself out which happens when its attitude changes somewhat. It acts like fuel starvation, like there is enough gas for idle but any more gas pedal and you get no gas to go with ithe increased air flow.

From searching threads the best possibility I have come up with is crud in the gas tank. I am going to check this as soon as the weather is nicer, but perhaps there are some other possibilities?

Ken964

Howdy Heirsh,

I have a similiar problem, feels like lack of fuel when I get below a quarter tank and on some downhills. A banked downhill to the right is the worst. I assumed it was some kind of crud in the tank but I have not taken it off to see yet.

good luck,

JasonAndreas

This is definitely not normal, there is another filter at the bottom of the fuel tank you also might want to check but if you do, don't reuse the sealing ring. (part #928-201-187-02)

WilsonAnderson

Heirsh,

I have the same issue with my C4. Bumps cut the "fuel" it appears while driving in my case. I did notice that the fuel level is ussually 1/3 or less when this occurs the most often.

I have taken the car to the Dealer (Beechmont Porrsche) Diagnoses: Poor seal around air filter, one coil was removed and re-installed cleaning the connections, the air flow sensor also had a (IIRC a green wire that was pulling out) that was re-inserted and "secured". Then the wires were looked after to make sure the connections were good there too. DME has been replaced with a new one and with a suspect one prior to taking to the shop. Same issue.

Yesterday afternoon after work the C4 (not what it was refered to yesterday) would not fire up and maintain idle. It would fire up, jump to 1500 rpm and settle down to 250 rpm and die after a few moments. Add throttle to the starting procedure and the rpms would jump up to 2500 and even 4000 rpm. I know bad for a cold start. Finally I was able to get it to run "fine" by applying throttle and praying. Yeah, actuall hands together prayer.

I am in the process of going over every electical connection I can find, bad ground maybe?

Please report what you are able to find.
Take care,
Wilson

Eric Kessel

Jason,
Can you get to that filter from under the car? Or do you have to remove the tank?

PaulR

Heirsch,

Have you checked the airflow sensor isn't sticking, and that the idle position sensor is releasing when you apply the throttle?

I have a similar problem, and believe it may be the airflow sensor sticking (thus the DME is not getting the signal that I've started to apply throttle) but have not yet ventured to investigate. I find it a very occasional issue, and not repeatable, so it's going to be difficult to know for sure when it's resolved.

I have also heard (but not witnessed, personally) that the potentiometer can be prone to wear and poor contact (after high mileage) in the near idle position, possible also contributing to such an effect.

...Paul.

JasonAndreas

Eric,
From the description given in the workshop manuals it doesn't look like you need to remove the tank. Its been 3 years since I've been down there to take pictures and I don't remember how accessible the filter is. When I had the tank drained and the pump replaced under warranty (10 years ago) the replacement seal was on the workorder and the Porsche mechanic did everything from underneath (except for siphoning, that was done from above) and never removed the tank so it should be possible.

Heirsh

Thanks guys. i hope to investigate some of these ideas this weekend. If I fix my problem I'll be sure to say. I'll probably check the air flow sensor first. Be easier to deal with than the gas tank for me.

Heirsh

Well i pulled the Air Flow meter this weekend. Not a lot I could do for it. It wasnt sticking, but I cleaned all the gunk from inside by spraying through with an electrical cleaner. I also popped the plastic cap to get at the electrical part of it. That is a piece of art there. Thankfully it was in good shape. I cleaned the outside plug connections, and used some electrical cleaner ont he inside, but otherwise didnt touch it. The car does drive better at very small throttle inputs so I guess I didnt ruin anything.

Bad thing is the problem with power less downhill still exists. I still plan to check the tank, but the more I think about it the more I just can't believe a gas tank obstruction could be doing this. More information on the problem.

If I blip the throttle going downhill the problem doesnt crop up, even on hills that always cause the problem.

I don't see how it can be related to the air flow meter, as it is before any vacuum that could be building it (all after the throttle butterflies). Also the vacuum would not be pulling the flap closed, it would be pulling it open, as it does under normal operation when the throttle butterflies open. So I'm thinking, if its related to vacuum, I need to be looking at things which use the vacuum between the butterflies and the engine. I wish my frickin repair manuals would get here. I don't see how you guys deal with this problem; it is driving me nuts.

Surely someone out there has fixed this problem as it seems to be relatively common. Fuel pump aging and cavitating at super low fuel consumption rates (this would, I guess, have to be in conjunction with a clog in the bypass as the pump should always be flowing right?). I don't know I'm grasping at straws. HELP!?

Mark Brook

Heirsh,
My car is suffering with the same problem, except it also does strange things
if you come off the gas too quickly. It's still in storeage at the moment, but my
mechanic has told me to look at the air filter/box, the ISV, air-flow meter and
lastly the Hall sensor. This is in the the distributor, and as yet I have no idea of
how to check. Not much help I know, but your not alone mate..........

Cheers,

Mark 90C2.

bhensarl

I'm picking up a trend here:
-Happens going downhill, especially in corners
-Happens with less than 1/2 a tank of gas
-Feels like fuel starvation
I had this same issue about a year ago, to the point that sometimes the car would actually die on me. I am convinced that it was as simple as condensation in the gas tank. These cars are 10-15 years old now, and I'll bet that in the last decade a good amount of sediment and other goo has settled in the bottom of the gas tank. Now combine that with leaving the car sitting in a somewhat humid locale for a week or so with a small amount of fuel in the tank and you have a recipe for poor running. But draining the tank completely and changing the in-tank fuel filter is probably a bit more than most of us want to do. Try this: fill the gas tank completely with high quality gas, put a bottle of Techron in the tank, and go for a nice long, spirited drive. And even if this doesn't solve your problem, you needed the gas anyway, and at least you got to go for a drive!
Brian

Eric Kessel

I have the same problem, with a low tank, always on the same slow hill in traffic on my way to work. It only happens if i'm coasting in gear, right when I put my foot on the gas. The car will buck (violently) for a few seconds.

This past weekend, I replaced my fuel pump due to it getting noisy (104k miles). I also drained the tank to clean the in tank filter. The filter was basically clean. It had collected a few small metal "shavings", but the size of the filter screen is very large. I posted pictures of the screen, and where it screws into the tank (take a look at this thread https://rennlist.com/forums/964-forum/192275-noisey-fuel-pump.html )

I felt around with my finger, and the tank didn't have anything on the bottom around where the filter screws in. I was hoping to find lots of crud to explain things, but I didn't....... Seeing this, I don't think i'd bother to clean the filter again.....

There must be a simple fix for this......

bhensarl

What about condensation, Eric? I was thinking the problem was actually more along the lines of condensation getting pumped through instead of fuel causing the power loss. After you've drained the tank and refilled it are you still experiencing the same problems?
Brian

Eric Kessel

I drained the last two gallons out of the tank into a new oil drain pan, and couldn't tell if there was water with the gas......... Also, while under the car I found two torn CV boots (both drivers side), so there than a quick couple test drives, I haven't taken the car to test out the downhill issue. I intend to replace the boots next weekend, and then try running around with the car low on fuel. The pickup is very low in the tank, so I don't think water would last long down there (water sinks below gas, at least I think so)

Heirsh

Well I finally got around to removing, checking, and cleaning the gas tank. I found a small amount of trash, but nothing to suggest the cause of this problem. Sure enough, after putting it all back together, the problem remains. I did not replace that little filter on the bottom, but it flows liquid no problem at all.

So I'm back seeking more advice/possibilities. I'm really at a loss. Anyone... ideas?