Rennlist - Porsche Discussion Forums - Progress on getting the new 931 started! And some ?'s

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Progress on getting the new 931 started! And some ?'s

I posted a couple weeks ago about getting the new-to-me 931 started, and I did get it started then, but was dealing with lots of white smoke, and bad idle/dying.

I've made some significant progress, as well as ruled out some things I originally thought were the issues.

I'm able to successfully and predictably get the car started once per day. It will idle around 2K for a minute before settling down to 1/1.1K. There's still some white smoke (not blue) but it seems to be either mild or moderate depending on the day. In addition, when I shut it off, I cannot start it up a second time that day (it needs to sit for a LONG time before it will turn over again). This makes me think that the head gasket is still an issue, because 1) the smoke and 2) I think the plugs are getting wet, and need to sit and have all oil slide off of them before it's clean enough for a spark again.

The car will idle for awhile though, which is good. This rules out the Fuel pump, 16A FP relay, and the overboost switch (currently bypassed as a way to rule it out).

At this point here's the main symptoms that still exist:
- smoke
- can't start again once it's been idling for awhile
- no headlights or interior lights (no headlight motor either)
- oil light is on
- gas light is on even though it reads full
- brake light is on no matter if the ebrake is engaged or not
- when idling, I have to put the gas pedal to the floor for anything to register
- pressing the gas causes the car to sputter/die rather than increase throttle. I'm assuming this is an air/fuel mixture in the carb

Any thoughts on those things that are still outstanding?

I don't know much about the 924 turbos... But I have a few comment for you. First, there isn't a "carb" - it's a Bosch CIS (continuous injection system) fuel injection. There's an airflow "sensor" which is a large flap that moves relative to air flow - when air flow increases, fuel delivery increases.
Second, if all of the warning lights on the dash are lighting up - oil, fuel, parking brake, etc. - then you might have an issue with the dash wiring (as opposed to all of those gauges not working). Is there an actual oil pressure gauge? If so, what does that show?
Third, you might have a major vacuum leak, and that's why there's no throttle response when you press the pedal.
Fourth, there's a temp sensor to trigger a "cold start" injector - this is to give it more fuel to start when cold. If it doesn't start when warm, something might be up with this system...
Fifth, any smoke, other than just a little condensation in the exhaust system, isn't good. I'd start by trying to investigate this by doing both a compression test and a leakdown test. Google or search YouTube if you need help doing that.

Thank you for your response! Yea I am currently learning about the fuel delivery system, and all the things you say make sense. I did a compression test, with these results:

Those seem consistent and good, which leads me away from a head gasket (but still doesn't account for the smoke & smell) so I guess the next step is a leak test.

Quote:

Originally Posted by mattaebersold (Post 15505741)

check auxillary air valve if it has one on top of engine. 924 Haynes manual is your friend here. Mine was bad once and sorta caused this type of thing with gas pedal. My 924 has one.

Also I would agree with cold start valve issue. I had one that leaked as I think most do over time. Replace those 2 parts with used junkyard / ebay items if possible.

I am guessing when headlights and interior lights dont work that taillights in middle rear dont work either. I had this issue, and it was to do with the switches on the doors that open/ground out when u open the door. They were old. I replaced both door switches and suddenly the interior and taillights started working. Be careful replacing these cuz you will blow several fuses if any part of the switch touches the body when removing or if rubber surround on them is dry rotted or missing. Even if you unhook battery first when battery is hooked up they will blow something. Driver door has 3 wires on it and passenger has 2 or something like that but they are exact same part. (Sorry its been 10 years since I looked at mine just trying to recall)

Headlights out may be the switch in the dash. They get dusty over time. Headlights should turn on without motor even and just will point light at the ground. Or inline with the switch behind dash follow wiring to a small black connector that may have a blown fuse in it. Its inline in the wiring on mine for some reason. Fuse blew once there on me and no idea why.

I am still thinking bad headgasket. Those are known for warping on 924s and I have replaced mine twice in 20 years at a qualified Porsche shop so I think it was done correctly. Fan switch in radiator fails prematurely and before you know it you are waiting for coffee in Taco Bell drive thru and white smoke starts billowing out of the hood. LOL

Maybe CIS issue too but I have never had issue with mine in 30 years and never touched it so knock on wood. I know they do need adjusting from time to time so I have heard at least.

remove that large black rubber air thingy coming off the CIS and bend and twist it looking for air leaks. Mine had a hairline crack you could only see if you took it off the car flipped it upside down and bent it in half and car would crank and crank but never start. Then suddenly it would start and run fine a few days. Then same thing again. I stuck some gorilla tape around it and problem solved then later glued it with hot glue gun and its held for years.

My understanding is that you can be burning coolant and still have good compression. I once had an old Toyota that ran strong but there was so much white smoke I got pulled over! Turned out to be a cracked block but nevertheless, I can see how a HG leak may not affect performance.

Sounds like you have a lot going on so maybe it's time to pull the head--and all the other stuff, including replacing / servicing sensors, afm, vac lines, etc etc etc

Though 120 seems a bit on the low side :(

Ok, and update!

It's running pretty smoothly when I can start it up, which is about once per day I believe that's due to the head gasket. There is some oil residue on the plugs, and I think the plugs get fouled and it needs to sit in order to clear it up. That said...

It idles high around 2,000 for a couple minutes, then it goes down to 1,000 and sits there for awhile. After that it starts to roller coaster a bit, like it dips, catches itself, and dips again, with increasing severity until the dips are so low that it dies.

A few things of note:

white smoke still visible, I believe this to be coolant getting in the combustion chamber (head gasket)
won't re-start for awhile (I'm assuming head gasket letting oil into the chamber/plugs)
really squeaky (see vid, possibly timing belt? Bearings around that?)
won't throttle past 2,000 starts to immediately sputter and die (vacuum leak?)
wavy idle to the point that it dies (vid, unknown)

VID

It kind of sounds like it's running on 3 cylinders... Can you put an IR gun on the exhaust headers right after it starts to see if one isn't getting as hot as the others?

I also wonder if it's becoming fuel starved - possibly a clogged filter or something up with the CIS system - and this is causing the hunting idle and failure to rev?

Do we also know if the valve timing is correct (timing belt marks at TDC)?

Agree with Van. Also, CIS cars are VERY intolerant of vacuum leaks/unmetered air. So you should be replacing all vacuum lines and checking all rubber parts in intake system for cracks, etc. If there is gummed up fuel, it could also affect the fuel distributor. I had to get mine rebuilt when I owned a 924 many years ago.

Awesome thank you! I will check these things

If it runs long enough to get the headers hot and you don't have an IR gun, you can just spray a little water on the header tubes to determine whether they are all getting hot.

On the 931, it is possible to install the fuel filter backwards as the fittings are the same on both ends. Obviously, the arrow faces forward toward the fuel distributor. Mine was in backwards when I first bought the car. There is also a strainer on the fuel pump inside the tank.

Daniel

'86 951
'82 931 (long gone)

Note that correct fuel filter orientation is indeed backwards - meaning, flow is to the rear of the car. Which is why it's easy to get backwards.

No header tubes on a turbo, it's a log style exhaust manifold. Needs a leakdown test, figure out if there's perhaps bent valves or headgasket or such.

I'm hearing a lot about bent valves... which is something I've never encountered on any other car I've owned. Are these motors prone to this? What could cause this?

Any car that breaks a timing belt or chain and is an interference engine (as this car is) is at risk of this. Timing belts (which this also has) are a lot more prone to breakage than timing chains, too. I haven't seen any evidence that this car is more prone to breaking timing belts than any other car, and the job on this one is relatively easy, but with older cars that weren't necessarily loved, it's not unreasonable to expect previous owners to have relaxed a bit on those belt change intervals.

Quote:

Originally Posted by mattaebersold (Post 15681817)

I'm hearing a lot about bent valves... which is something I've never encountered on any other car I've owned. Are these motors prone to this? What could cause this?

Ah that makes sense! I've never had an interference engine before, so that might be why that has never come up in any discussions. Thank you!