CIS Pressure testing

[mikemdd]

I have an 81 SC that has what I've seen as a common cold-start problem. When cold, it will start right up, but stumble badly for about 10 seconds, sometimes with very mild backfire, and then either smooths out or dies. It then starts back up with no problem. Alternatively, I can start it up, immediately shut it down and restart with perfect results. It sounds like a fuel pressure problem.

As background, I tested and adjusted the mixture with an LM-1. I have the AFR set at 13.6, or around 2.4% CO. During warmup from cold, it starts at an AFR of 11.6, gradually increasing to the final warmup value of 13.6 after about 5 min. Looks like it is doing what it should. It runs great.

Second, I checked the fuel pressure values...system pressure, cold pressure and warm pressure. Everything was right on the spec. value. No problem here.

Finally, I checked the residual pressure. Interestingly, it had the following values:

- 3.4 Bar pressure just before turning the engine off
- Pressure immediately dropped to 2.55 Bar when engine turned off
- Pressure after 4 min = 1.8 bar
- Pressure after 20 min. = 1.6 bar (spec. is greater than 1.1 bar)
- Pressure after 3 hour = 1.5 bar
- Pressure after 6 hour = 0.2 bar

As noted above, the residual pressure is within the spec after 20 min. However, I'm wondering about the immediate drop from 3.4 bar to 2.55 bar right after shutdown. My question: even though the residual is in spec, could the immediate drop from 3.4 bar to 2.55 bar after shutdown be my problem? Has anyone else checked their pressure with a similar immediate drop? Thanks.

Mike

[RarlyL8]

Pressure is held by the fuel accumulator. I don't know the exact bleed-off scale but do not believe it should be so sudden.

[ianc]

There are a couple things that can affect startup, and cold running immediately after startup.

If the car starts right up and runs, the AAV and cold start valve are probably working fine.

If it starts, but stumbles thereafter, it's probably the WUR or Auxiliary Air Regulator.

If you are certain your cold control pressure is within spec (WUR controls this), then your WUR is probably working properly.

Take a look at your AAR to make sure it's getting juice and is able to open properly.

ianc

[sammyg2]

The injectors are supposed to close at around 3 bar, so the pressure should drop to that point immediately. No clue why it would drop farther.... .but that shouldn't have anything to do with a cold start problem.
Have you tested your AAV? sounds like it isn't supplying enough extra air for a high idle, or maybe it is supplying the air all the time and you adjusted your warm idle to compensate?

Darn, too slow. Ianc beat me to it.

[mikemdd]

I replaced the AAR last fall with a new unit (very difficult to find the correct AAR for an 81 SC!!) Also, it fast idles as it should, with the idle slowing as it warms to the warm idle point. As was noted, the WUR seems to be fine, both by pressure testing as well as actually measuring the AFR during warmup (rich to start, leaning out to 13.6 at full warmup). My thinking was possibly two problems. One was that the injectors were leaking slightly. However, I never smell fuel, and starts after the initial cold start are completely normal. The second possibility was fuel pressure. When warm, there is never a problem. However, the fact that simply starting the car when cold twice fixes the cold start seems to indicate that it takes a little while for the pressure to increase. It's like the cold start valve gives it the initial "boost", but then it runs out of fuel until the pressure builds up?? The only strange reading was how fast the initial pressure dropped after engine shutdown. Maybe it is the accumulator...it holds at lower pressure, but will not hold at higher pressure. Anyway, I was curious if anyone else has tested with gauges and saw the same thing. Ultimatley, it may have nothing to do with the cold start issue. Interestingly, the problem just started after sitting over the winter.

Mike

[Jim Williams]

[QUOTE]As noted above, the residual pressure is within the spec after 20 min. However, I'm wondering about the immediate drop from 3.4 bar to 2.55 bar right after shutdown. My question: even though the residual is in spec, could the immediate drop from 3.4 bar to 2.55 bar after shutdown be my problem? Has anyone else checked their pressure with a similar immediate drop?[QUOTE]

The Porsche CIS trouble shooting guide says that "at first the pressure drops fairly quickly to about 2 bar. It should then drop slowly. After 20 minutes there must be a resdual pressure of 1.0 bar." This book covers up through the '76 MY, but there is no reason to think the check would be much different for the later models.

There are several places the pressure COULD leak down:
WUR
Cold start valve
Accumulator
Fuel pump check valve
Pressure regulator in the FD
Throttle CPR (up through '75)
WUR check valve in fuel distributor (later cars, don't remember off-hand which year this was added to the fuel distributor)
Frequency valve

Another possibility is the air flow sensor plate set too low. this is also mentioned in the trouble shooting guide.

[ianc]

I can't see how a low residual pressure could cause this type of difficulty though. If anything, I should think it would cause a hard or no start condition, but not poor running after starting...

ianc

[mikemdd]

ianc,

I think what is happening is that the cold start valve is supplying enough fuel during cold startup to get it to fire up, but the assumption is that there is not immediately enough pressure to keep it running. Again, if I immediately shut it off and restart, it runs perfectly when cold. Perhaps the pressure is not building fast enough...weak fuel pump, clogged fuel/accumulator, etc. Anyway, compared to other's problems, mine are relatively minor, in the scheme of things.

Mike

[Paulporsche]

What idle speeds are you getting during warmup? Once fully warmed up?

Did your CCP fall between 1.35 and 1.8 bar @ 10C?

Have you tried a richer mixture, say up closer to 3.5%? If you richen it, you will probably have to increase the idle speed.

You said you had 3.4 bar RESIDUAL pressure just BEFORE turning the engine off. You should have 4.5 t0 5.2 bar. FUEL PRESSURE> Did you mean just AFTER? Or did you mean CONTROL pressure? That should be 3.4 to 3.8 when warm.

[mikemdd]

Paul,

Good questions.

1) Idle speed

- Idle starts at around 1400 rpm cold, then drops to 900 when warm (about 5 minutes or so). Again, this is after the second start cycle, where the idle immediately acts normal.

2) CCP

- The cold control pressure, measured at 59F/15C (coldest ambient temperature available after sitting for a couple of days), was 1.65 bar. Spec. for for my car at this temperature is 1.6 to 2.1 bar.

3) I have not tried a CO% greater than the current setting of 2.4% or 13.6 AFR.

4) The SYSTEM CONTROL PRESSURE was 4.8 bar. Spec. for my car is 4.5 to 5.2 bar. The 3.4 bar pressure I noted was while the car was running and represented the WARM CONTROL PRESSURE. Spec for my car is 3.4 to 3.8 bar. This was the pressure before turning the car off.

As you can see above, the idle, CO%, System Control Pressure, Cold Control Pressure and Warm Control Pressure are all within spec. Perhaps I can try to richen it up from 2.4% to 3.5% to see if this makes a difference. However, given that a quick second start when cold fixes everything tells me that it has something to do with getting the pressure up during intial startup...but then maybe not.

Mike

[ianc]

I think what is happening is that the cold start valve is supplying enough fuel during cold startup to get it to fire up, but the assumption is that there is not immediately enough pressure to keep it running.

Hmmm... Sounds a little shaky to me. You could test this theory by jumping the safety switch on the back of the throttle body, then letting the fuel pump run for 10-15 secs or so to build up pressure and see if you have the same problem. I don't think it will help though...

ianc

[mikemdd]

ianc,

Your idea is good. I think tomorrow I'll give it a try and let you know. Should be a definitive test for the pressure idea.

Mike

[Paulporsche]

Mike,
It's a quick, easy thing to try a slightly richer mixture. Turn your idle screw too when you do this.

[mikemdd]

ianc,

You were correct. This morning I jumpered the relay to run the fuel pump prior to cold start to get pressure in the system. No improvement. Again, it fired right up, but then stumbled as usual. I only let it stumble for about 3 seconds...turned it off and restarted. As usual, perfect cold start idle and warmup after that. It literally only takes a few seconds on the the first start before turning off to fix the problem...very strange.

Paul: I will go out and richen it up as suggested and let you know. If the idle changes, I'll readjust to around 900 rpm. I will obviously have to wait to check the cold start.

Thanks to all.

Mike

[mikemdd]

OK...adjusted AFR/CO% to 13.2/3.5% per suggestion. After sitting all day, did a cold start with no improvement in stumbling. Second start perfect. I don't know how a simple second immediate start can make so much difference. I'm out of ideas.

Mike

[brett25]

hi mike,

my car does the same thing. Not to often. Maybe once in 5 cold starts. It may have a relationhip to amount of gas in tank. I try to keep at least half a tank if I can.

I just let her go through her thing, then were off......

[ianc]

Have you checked the obvious things like valve adjust and timing? Vacuum advance functional?

If so, it still seems like it's got to be fuel to me. On the second kick you get a second shot of gas from the CSV that may just allow it to run smoothly until it warms a little...

ianc

[mikemdd]

Thanks everyone for your suggestions. I think I'll just accept the cold start idiosyncracy and be thankful it is perfect in nearly every other way. I will continue to think of other ideas and will let everyone know "when" I find the solution. Cheers.

Mike

[RarlyL8]

Mike,
This might sound a little lame but in a system that is 20 years old (give or take) every component is likely to be worn just enough to remain in tolerance but as a whole perform less than optimally.
A worn air sensor plate bearing coupled with the odd injector misfire or spray pattern....... you get the idea.

Sometimes you have to wait for a failure before finding the true culprit, given there is only one.

[Mysterytrain]

I was reading this 'old' thread [ my car also does this] and I had some thoughts. I think in my case it might be more temperature dependant. A couple of things happen at start up..you turn the key and the starter circuit spins the motor and fires the cold start valve. The AAV provides a big ol' gulp of air and then closes [vacuum pulls it shut]. The AAR is open enough to provide a high idle [which might be high enough to advance the timing??] Vacuum to the WUR [78sc] is blocked by the thermotime valve..once the valve opens [heating element heats up after 8 secs or so] vacuum is applied to the WUR and system leans out. My thoughts are that this condition occurs after the cold start circuit turns off and before the thermotime valve opens and leans out the WUR. So it kinda points to the cold WUR setting- Maybe? My car does the same as Mike states his car does..if I fire it up and let it cough..then shut it off and restart..it does a perfect warm -up. Odd one isn't it. Now, I think my car has a much smoother start up when its colder outside. Again, the two temp dependant devices are the WUR and the ol' AAR. The AAR's opening is much larger when the ambient temps are lower. So the engine is rev'ing faster at start up [air sensor plate is lifted and more fuel is flowing]..and we are out of the idle circuit mixture settings [I'm guessing here -no? yes?] Anyway, I'm thinking its a mixture issue during that brief window of coughing..but is it too rich or too lean? and is it the WUR cold pressure or the mixture adjustment?