Help with CIS Again - Hot start, changed everything...any ideas?

[Iciclehead]

I know, just what we need, another thread about recalcitrant CIS.

Engine is 3.2 SS, built by Kremer in Germany, stock Euro CIS, SSI's/standard muffler. It has been starting, running and behaving perfectly for 10+ years, only minor issues.

Few weeks ago, driving my mom around of all places, stopped for a few minutes and the car wouldn't start. It spun with great energy on the starter (not an issue with battery), healthy whine from the ignition but would not start.

Finally got it going by lifting the air plate with the switch on, got the fuel pump running and it sputtered to life. Ran like a clock for the 2 hour drive home. Stopped for gas, same thing. If I let it sit for an hour it starts fine (likely as the cold start circuit is engaged).

Well, researched the archives, found out that it could be a fuel pump, check valve or accumulator issue.

Replaced the fuel pump and check valve first. No effect, still fail on hot starts

Replace the accumulator, albeit with the one recommended by Bosch which is considerably smaller in volume than the one that was in there), still 3 port so it fit OK.

Tried it a few times, seemed to work and hot start just fine. Went on a 600 km road trip, wouldn't you know it....failed when I stopped and continued to fail since, same failure mode, starter spins, let it cool down and it is fine.

So I see three potential issues that are left:

1. One of my new components is faulty. I would suspect the fuel pump is fine as the car runs well once started, so perhaps the accumulator or check valve

2. The smaller accumulator that Bosch currently spec's for a '79 CIS 911 is not correct and I really need the larger one (big version was Bosch p/n 0 438 170 031,smaller version p/n 0 438 170 009). From everything I can find, it cross-references and ought to work properly regardless...it just has a smaller volume, same principles of operation

3. Something else is wrong. Sort of doubt this as the car runs perfectly once starting, good power, fuel economy, cold starts, idle....couldn't be better, but hey....I will happily bow to the better knowledge of others

Help?

Dennis

[stlrj]

Originally Posted by Iciclehead

Finally got it going by lifting the air plate with the switch on, got the fuel pump running and it sputtered to life.

It might be a bad thermal/ time switch. Seems your engine is not getting that extra spritz of gas it needs from the cold start valve to fire off the warm engine when you crank it over. Explains why pushing up on the sensor plate works when the cold start valve should be doing that for you.


Cheers,

Joe

[Iciclehead]

Originally Posted by stlrj

It might be a bad thermo time switch. Seems your engine is not getting that extra spritz of gas it needs from the cold start valve to fire off the warm engine when you crank it over. Explains why pushing up on the sensor plate works when the cold start valve should be doing that for you.


Cheers,

Joe

Thanks for the thought, I will check my documentation and see where the thermotime switch cuts in. I thought it was only in cold starts, but let me dig into it.

My presumption in pressing up the sensor plate was that it would start the fuel pump up and pressurize the system in spite of the accumulator not holding pressure.

Dennis

[allaircooled]

When I had similar symptoms I made sure it wasn't the three you listed first and it turned out to be the o-ring on the primary pressure relief valve.

[boyt911sc]

Originally Posted by allaircooled

When I had similar symptoms I made sure it wasn't the three you listed first and it turned out to be the o-ring on the primary pressure relief valve.

Aside from the commonly known culprits (FP check valve & FA) for residual fuel pressure loss, your FD has the primary and return valves. This is the third culprit to check if the residual pressure is prematurely getting too low.

Tony

[stlrj]

Originally Posted by Iciclehead

Thanks for the thought, I will check my documentation and see where the thermotime switch cuts in. I thought it was only in cold starts, but let me dig into it.

My presumption in pressing up the sensor plate was that it would start the fuel pump up and pressurize the system in spite of the accumulator not holding pressure.

Dennis

Thermal/time switch is stamped 45 degrees C which feels quite warm for a cold engine IMHO. The popular misconception is that the cold start valve only operates on a stone cold engine.

Joe

[T77911S]

a hot car will start without the CSV. cold ones can too, just takes longer.

the system pressure regulator has a pull up valve in it that closes the return line from the WUR to the fuel tank. (along with closing the return for the primary pressure). this helps to maintain a higher residual pressure, and perhaps longer.
i would check residaul pressure, then pull and replace O rings on the sys pressure reg then check pressures again. be careful. there are some small parts on the pressure reg.

you can try pulling the connector off the back of the AFM so the fuel pump runs with the key on. this may help til you get it fixed. this is a safety feature put it to stop the fuel pump if you get in a wreck and the engine stops running but the key is on.

[76carrera3.0]

As Tony said, plus the WUR must also be able to hold residual pressure when the car is shut off.
You need to get a test gauge on it and work through the tests to isolate the component that is NFG
Otherwise you are gonna spend big big money on parts you don't need. With the gauge and tests done you will just spend big money to fix it . CIS parts are crazy $$$$$$ now if you can even find them to buy.

[Iciclehead]

Originally Posted by T77911S

a hot car will start without the CSV. cold ones can too, just takes longer.

the system pressure regulator has a pull up valve in it that closes the return line from the WUR to the fuel tank. (along with closing the return for the primary pressure). this helps to maintain a higher residual pressure, and perhaps longer.
i would check residaul pressure, then pull and replace O rings on the sys pressure reg then check pressures again. be careful. there are some small parts on the pressure reg.

you can try pulling the connector off the back of the AFM so the fuel pump runs with the key on. this may help til you get it fixed. this is a safety feature put it to stop the fuel pump if you get in a wreck and the engine stops running but the key is on.

Good idea for the temporary fix...right now I am fearful that it won't start when I go get gas.

I am going to check out the thermotime switch and cold start, partly as I have spares for those and can swap out.

I presume there is nothing special about the O rings that are used for the system pressure regulator....just the proper size and all will do it? They aren't Viton or something special that would make me need to go to Porsche to get the right magical material?

Thanks all, this is quite useful. I would have never thought of the temporary fix on my own....

Dennis

[Paulporsche]

Have you checked your control pressures and your mixture setting?

Could be they are off for warm/hot conditions. Often the CO is too rich for warm starts.

Vac leaks are always possible for these cars, but usually they are are a problem from cold rather than hot.

[Iciclehead]

Originally Posted by Paulporsche

Have you checked your control pressures and your mixture setting?

Could be they are off for warm/hot conditions. Often the CO is too rich for warm starts.

Vac leaks are always possible for these cars, but usually they are are a problem from cold rather than hot.

Actually I don't have a gauge set...sort of counting on the fact that the car runs perfectly other than the hot start issue, would suggest that there are no issues with leaks and such...but if the problem does not get figured out soon, I will undoubtedly need to get the gauge set and just get on with it.

Sigh, and thanks...

Dennis

[stlrj]

Originally Posted by Iciclehead

Actually I don't have a gauge set...sort of counting on the fact that the car runs perfectly other than the hot start issue, would suggest that there are no issues with leaks and such...but if the problem does not get figured out soon, I will undoubtedly need to get the gauge set and just get on with it.

Sigh, and thanks...

Dennis

I doubt you will need gauges to solve this either.

[T77911S]

Originally Posted by Paulporsche

Have you checked your control pressures and your mixture setting?

Could be they are off for warm/hot conditions. Often the CO is too rich for warm starts.

Vac leaks are always possible for these cars, but usually they are are a problem from cold rather than hot.

mixture can do it too. along with plugs, cap and rotor. when my brother had my car he replaced the WUR which fixed it when he had it. (control pressures)

i would want to know what the pressures are doing.

when i sold my 77 it was not holding residual pressure. for me it would take about as long as it took to eat lunch. since my FP ran all the time i would let it run for a minute, hold my foot to the floor and it would crank. sometimes i might have to lift the sensor plate.

some tools are just a good investment. gauges is one of them. a hand vacuum pump is another. (something most dont think about).

[stlrj]

If you're worried about not having enough residual pressure, you can always verify that without gauges. Just put your hand under the sensor plate after the engine has been off for 10 mins and feel if there is any resistance when you push up on the plate. Resistance means residual pressure is present.

Again, no gauges required. Just common sense.

[Bob Kontak]

Any one of the fuel injectors could dribble and kill your cold start ability.

Your cold start valve can leak as well. It is not fun to pull out but you can pull back the CSV while connected, lift the air sensor and see if it bleeds.

Mind you, smarter CIS guys know where to prioritize this risk. However, if you pull one injector out at a time, "blip" the air sensor (wee tiny bit) and then check for drips you can put this one to bed.

I have metal fuel lines on my 81. I can rough-house them more that flexi-lines. I don't think any pull pressure can be put on your lines if you are remotely conservative.

Please see post #2 where I said to yank on the hose and then post #6 where Tony respectfully handed me my rear end.

Tool AVAILABLE: TO REMOVE CIS FUEL INJECTORS...?

[Joe Bob]

A 3.2 is not CIS......

[Iciclehead]

Originally Posted by Joe Bob

A 3.2 is not CIS......

Oh yes it is when it is a Kremer built 3.2 SS (short stroke). Engine was originally a 930/03 (Euro 3.0 CIS), updated at the time of rebuild with new 98mm P/C's. I actually got the Euro 9.8:1 compression ratio, so I need to be pretty careful on fuel quality.

...quite a nice engine actually, nice power band, fun to drive and good in terms of fuel economy.

Dennis

[Joe Bob]

Then CIS is all about residual fuel pressure while hot. Check valve in the fuel circuit is the cheapest fix.

[Iciclehead]

Originally Posted by Joe Bob

Then CIS is all about residual fuel pressure while hot. Check valve in the fuel circuit is the cheapest fix.

Yes, I did that basically first. I actually was hearing my fuel pump from time to time and thought it good insurance to replace the pump and the checkvalve at the same time. Those changes had no effect.

I then swapped out the accumulator, albeit with a smaller volume one which is the current Bosch spec, it seemed to work a bit at least for a few trial runs, but then it failed as per my original post.

Issue is now where to go.....it is clear that it is losing residual pressure, question is where and I was hoping to avoid the cost of buying the test set as the system has been so blessed reliable that I figured I would never need it. Until now.

I also have a complete spare injection system, so swapping parts is possible....given those are working of course.

Dennis

[Iciclehead]

Originally Posted by stlrj

If you're worried about not having enough residual pressure, you can always verify that without gauges. Just put your hand under the sensor plate after the engine has been off for 10 mins and feel if there is any resistance when you push up on the plate. Resistance means residual pressure is present.

Again, no gauges required. Just common sense.

Good thought and I will try it.

Puts in mind of the commentary made by the last Porsche mechanic at the local dealership who understood CIS. He checked my set up a few years back, told me it was basically the last time anyone at Porsche could do it as he was retiring and the new mechanics have neither the skill, time or interest in dealing with a system as old as CIS. When I expressed worry about how to keep the mixture in tune, he said that was pretty easy. He said get the car warm and idling, gently lift the sensor plate a little (increasing fuel flow without increasing air flow) which will make the mixture richer. If the engine increases in speed then it is running a little lean. If it gets rough and crappy, then then it is probably a little rich and if it just purrs along it is likely just right.

His experience was that the CIS systems run forever unless a component fails (and there are lots of them) and they hold their settings quite well. He figured don't dick with it until there was reason to. I like him a lot and I figure 40 odd years experience with Porsche qualified him to say that.

Anyway, thanks for the idea...it will get tried.

Dennis