Porsche 944 - only stays running via throttling

[bryske]

Hi "pelicans" :-)

I hope for a little guidance.
I've got a Porsche 944 (late 1985, 8V), that only can run if you constantly are throttling the accelerator pedal. It won't run in Idle by itself. And if you keep the accelerator at same level it also dies. You need to keep pumping the throttle in a systematic way to keep it alive. It seems to be something like. No throttle (1 sec) - 75% throttle (1 sec) - No throttle (1 sec) - and so forth.

I've tested via Clarks garage instructions:
DME temp sensor -> OK
AFM -> OK
Alternator and battery -> OK
TPS -> replaced (I though it was the problem) -> OK
Fuel pressures -> OK (29 PSI and 36 PSI)
Ignition wires + spark plugs + ignition cap + ignition rotor -> replaced -> OK
Idle screw adjustment -> OK

What's next to do? And also if you got inputs on how to do, it's appreciated.

Didn't test ISV yet, don't know how to do it?

Thanks for any help in advance.

[Monkey Wrench]

puping the pedal to keep a motor running is often a throwback to carborated cars that had an accelerator pump. In those cars doing that produced more fuel and helped in situations of fuel starvation often during warmup.

I don't know why you are seeing this but I suspect you have an opening or leak somewhere in between the air mass meter or fuel measuring device and the manifold, that could be a hose that's come off or has seen better days. i think it just wonr run at low RPM's but you are working around that by increasing the revs periodically and that keeps it going but it's not right or acceptable.

If air finds its way in some other route then it isn't measured by the AMM ( or barn door) so no fuel is injected to compensate and the engine runs lean, this is more apparent at idle than full throttle. some inject smoke or use a vape pen and a small compressor to charge the area with "smoke" and look with a flashlight for any place it's leaking out. I'd suggest that. the system is very sensitive to this sort of thing so its a good place to start.. also think back if you have changed anything sometimes that can point to some trouble caused just previous to the issue.

I work more on various equipment than cars but when faced with a difficult issue I always like to ask questions like did this just creep up on you or did it come on suddenly? any other factors you can think of that may have caused a sudden change? what do you think the problem is? asking that usually helps. They may have no idea of the issue but often they have some info, maybe buddy dropped a wrench in the works and they can have an opportunity to say this happened but dont tell the boss;-) I have them test and show me the issue, then I fix their machine , then I either demonstrate or have them demonstrate to prove Ive solved the issue I was called upon for, that goes a long way as to not have return calls , especially for unrelated issues.
often if someone works on your car and you have a service advisor between you and the mechanic, it can prevent good conversations.

a test you can do is with the fuel pump, you can check the flow rate, make sure the fuel is sufficient, gas tank screens can plug. not enough info to nail it on that but you can check the fuel pump flow rate.

bad gas can happen if car was stored or recently filled, you can siphon some off and renew most of the gas if you think that's a possibility, once issue is resolved you can burn the gas, water in your fuel can cause problems.

i was just recently having poor idle especially when cold in my van but even when warm it just ran and missed and ran rough. I duped a whole bottle of sea foam in the tank and the problems magically disappeared, problem with a stubborn injector? maybe? I was about to clean the distributor cap and plugs, you might do that, and look at the ends of your cables, look for any green deposits like if water got in there and caused corrosion. the caps with aluminum electrodes get bad faster, usually you can take a sharp knife and clean any gunk off as a temporary measure, the OEM cap might be a bit expensive but I dont trust the aluminum type ones very long.

I dont think the rotors fail so easily you can clean the end of them. sometimes bad plug wires spark to ground instead of firing the plugs, you may see that in darkess , id just say give all the plg wires a wash with a bit of alcohol once a year if they seem old replace them. pull your plugs have a look, are they even? do they stink of raw gas? are they wet? they should be tan in color are they all black or white or any of them? if they were dirty you might need to clean or replace, run and then remove to see the changes. the condition of the plugs says a lot as far as fuel ratio, or may lead to other issues like a stuck injector dumping fuel in after shutoff.

you can pull one plug wire at a time, any difference, that shoudl make it run worse, unless it happens to already be trying to run on 3 cylinders. you can do similar by unplugging each injector and starting between, just briefly to note any differences . there is a thing you can buy called a "node light" it will plug between the injector and cable lead and flash when the injector fires, it proves the injector has it's pulsations.

its possible to pull the fuel rail and check injectors to see if they spray about the same and shut off after key off and that they arent; stuck shut, you can test them on the fuel rail or remove them and do it in your own shop, or send them out or install rebuilt ones, but be careful not to start a fire if you have fuel around, go find a proper fuel rated extinguisher before you open a fuel line and best to take the car outdoors just in case.

I know a guy that drained gas into a bucket to clean his tank, and tripped on his trouble light and his life was never the same after his face was all burned, and he lost his shop, so yea don't do that. please be careful.

others will direct you to the garage site and there are some rational sequences you can follow so that you are troubleshooting in a less random order, this is better.

plug in connectors and grounds can cause grief so you are smart to go around and clean every ground possible and maybe check the connectors, look carefully at wires like the ones that go near the bell housing to sensors, they can get oil impregnated hard and suffer from vibration, causing all sorts of unexpected grief, best to address that before you need to troubleshoot things.

I'd start it between anything you do, even if it still runs like crap, so if and one your changes do magically just improve, you have a target area in mind. same goes if you go swapping sensors at random. try not to change 6 different things at once or you can have a car that wont start for three different reasons at once, no one deserves that ;-)

if the timing belt on this car breaks, it can cause a valve crash because it IS an interference engine. I'd check how old it is. You really do not want that issue on your hands. If its backfiring or doing other weird stuff you may stress it in some unusual way and cause a failure if it's old. if the fuel pump has never been changed you are likely due for a filter and a pump and a fuel tank screen. some like to replace all the fuel lines also, a leak over the manifold can cause an engine fire.

the fuel pressure regulator has a small hose to sense vacuum, pull that hose off, if there is any gas in that hose it's a bad fuel pressure regulator.

did the car run fine until recently when you had this issue, or was it parked a long time? did the problem arise suddenly or gradually?

look for any hoses that may have come off including any smaller hoses in the engine bay. you might just have a hose that's broken or come off and has been unnoticed.

if that wont hep look over "clark's garage" notes. You'll find links if you search around this site a bit and also old threads with similar issues.
you may need to hunt a little but the OEM service manual is floating around too. It will have a lot more specific info. I dont think Porsche released the copyright officially so it may not be online exactly but if you search you'll get a copy or you can buy an original manual and get the paper one.

I think it actually benefits Porsche to release the for 35 year old cars because seeing them on the road,and vintage, is good for current business. I'm obviously not in charge of all that.

Please post your findings, others here will help and some know a lot more than I do. Posting the resolution helps others. you'll find a lot if you go to the directory of posts in this particular Porsche 944 forum and then use the search box, a lot of this is just repeating that which was previously covered. look down each thread and try to see the resolution, that will give you stuff to think about.

try to approach it systematically. it seems like you are running lean, the plugs should prove or disprove that, Id start there. just clean and observe at first. dont go crazy swapping parts hoping for a random win, but do replace old worn parts especially those known to fail and leave the car in a no start situation.

[Jfrahm]

How do the new plugs look after running like that? Wet? Black? OK?

[walfreyydo]

Hmmm I dont see vacuum leak on your list.

Dont underestimate the degree to which a vacuum leak can cause running issues on these cars. They are also extremely elusive to locate.

Smoke test, vacuum gauge, slightly pressurizing the intake are all methods to identifying this issue. Lots of general information on youtube which could apply across makes and models as well. I had the vacuum line disconnected from under the intake once and the car would not run or idle. Reconnected and the car ran perfect.

Info on the ICV:
https://www.clarks-garage.com/shop-manual/ts-01.htm (see engine idle surges section)
https://www.clarks-garage.com/shop-manual/fuel-21.htm
https://www.clarks-garage.com/shop-manual/eng-15.htm

O2 sensor another possibility but I doubt thats the issue.

[bryske]

Hi all,

Thanks alot for all the help and good advice. It's really appreciated.

I toke up the "smoke test" advice first, since I realized I didn't do that. I had done a "visual" inspection of the vacuum hoses, and did the stupid assumption that was well enough. But the smoke test revealed the one hose I hadn't seen. The one below the intake manifold, coming from the ISV and going to the intake manifold. I couldn't see it at all from above.
Well the smoke test revealed it for sure!! :-D . I think the culprit are found or at least my fingers are crossed!.

I'll attach a few images:

https://ibb.co/LQ2XM3T
https://ibb.co/M5X4Jx4
https://ibb.co/kGZ4kSY
https://ibb.co/bsM4gLm

Now I've order the hoses and a few other spare parts. Eagerly awaiting their arrival to test if it fixes the problem. :-)
I'll return with the result next week when the parts have arrived.

Once again thanks for all your valuable help.

[Monkey Wrench]

Great! that looks significant and pretty simple to fix once you have some hose that's appropriate.

[djnolan]

i would have told you to check that first. Way to go!