1974 CIS hot start issues

[Nate W]

Hey Pelicans, I have a 1974 2.7 CIS engine in my 1971 914. It runs great other than it doesn’t like to start when it’s been running in the heat. I know this is a common issue, and I have tried to do some work as far as testing but thought I would ask for some help since this problem is perplexing to me. The car has had in the last year:
New air box no pop off valve
FD rebuilt
WUR rebuilt
New injectors
New fuel lines (car)
TPS and cold start valves inspected for function
New rubber peanut boot
New FP
New FA
New filter

I hooked up the fuel pressure gauges to the FP , it will hold 88 PSI for 15 minutes so I figure good
FA will hold 27PSI for 15 minutes, also seems ok
When I connect gauges between FD and WUR, it will lose pressure in about 1 minute. Also when I turn off the valve, eliminating the WUR it still leaks

I have tried playing around with the sensor plate height a little bit but no matter what it seems to be leaking, which I’m pretty sure is what leads to the hot start issue since I can’t keep the fuel pressure in the lines.
Is there anything else I can try to check? I really appreciate any feedback, kind of new at this so looking to learn! This board is an awesome resource for me already
Thanks
Nate

[4sd911]

When the engine is hot and hard to start does it freely turn over or does it crank slower? If it cranks slower what does the oil temp gauge on the dash read?

[dailydriven911]

How is your battery and battery cables? I'm having the same issue and I have heard this can can be related to the battery and/or the cables. Just an idea

[PeteKz]

Did this problem occur suddenly, or did it get worse gradually? Don't mess with the sensor plate yet. That will upset your other troubleshooting. If you think you are losing residual pressure, find that first.

I wasn't clear how you measured the pressures. The standard procedure is to install your gauge/valve setup between the FD and the WUR, with the valve towards the WUR, not the FD.

What is the System pressure with the FP running (valve closed)? What is the warm Control Pressure (WCP) with the FP running (valve open)? Then turn off the FP and read how long it holds residual pressure (RP). Don't just stop at 15 minutes, but keep tracking it at least 30 minutes, preferably an hour.

When the car is hard to start (and I will make the distinction here that it IS cranking normally, just not firing--correct?) spray starting fluid in the intake. If it starts right up, then you are not getting fuel. This rules out ignition problems.

What is the number on your WUR?

[Nate W]

Originally Posted by 4sd911

When the engine is hot and hard to start does it freely turn over or does it crank slower? If it cranks slower what does the oil temp gauge on the dash read?

No it cranks normally, I didn’t see the gauge do anything other than return to the temp , not pegging or anything. Thanks

[Nate W]

Originally Posted by dailydriven911

How is your battery and battery cables? I'm having the same issue and I have heard this can can be related to the battery and/or the cables. Just an idea

Thanks but brand new AGM battery and cables, grease, etc.

[Nate W]

Originally Posted by PeteKz

Did this problem occur suddenly, or did it get worse gradually? Don't mess with the sensor plate yet. That will upset your other troubleshooting. If you think you are losing residual pressure, find that first.

I wasn't clear how you measured the pressures. The standard procedure is to install your gauge/valve setup between the FD and the WUR, with the valve towards the WUR, not the FD.

What is the System pressure with the FP running (valve closed)? What is the warm Control Pressure (WCP) with the FP running (valve open)? Then turn off the FP and read how long it holds residual pressure (RP). Don't just stop at 15 minutes, but keep tracking it at least 30 minutes, preferably an hour.

When the car is hard to start (and I will make the distinction here that it IS cranking normally, just not firing--correct?) spray starting fluid in the intake. If it starts right up, then you are not getting fuel. This rules out ignition problems.

What is the number on your WUR?

WUR is an 006 model
I installed the gauge first between FP and Accumulator to test pump, closed valve, it held pressure 88 PSI for 15 minutes when I stopped the test
Then I installed the gauge between accumulator and FD, turn on pump for 10 sec, close valve. It was holding at 27 PSI at 15 minutes when I stopped the test which should be fine I believe.

Then I moved the gauge to the normal testing position as you mentioned, valve on the WUR side. Turn on the pump with valve open (so WUR is included in the test ), turn it off and it loses pressure in about 1 minute.
After that I turn the pump on again,closed the valve so the WUR shouldn’t be included, and it still loses pressure , which makes me think the FD is possibly leaking between supply and return?

[Nate W]

I did some testing on a cold engine, ambient temp was 83F
WUR was unplugged
Gauge between FD and WUR, valve on WUR side of gauge

With only fuel pump running, valve closed I get 40 PSI

When I open the valve to WUR I get 36 PSI

When I plug in the WUR I get 38 PSI

It’s the engine running I get 40 PSI, goes higher like 45 when engine RPM goes up

I didn’t want to drive it with the gauge hooked up, but let it idle for about 35 minutes to get it up to 210F on my dash gauge which is about typical temp for this car, ( to simulate hot start conditions). When I turn off the car, the residual pressure leaves almost immediately about 1 minute from 40 PSI to 0. My understanding ( or maybe lack of) is that the lack of residual pressure would be a cause of the no start symptom when hot. It will start if i do it immediately but if i wait about 10 minutes it won’t want to start until it cools down a lot

[PeteKz]

Good info. I agree that your FD is leaking pressure, and that your FA and FP check valve are good. Yes, loss of RP will cause hard warm starting, in fact that is the usual cause.

Your SP is very low at 40psi. The SP should be between 4.5 and 5.1 bar, or 65-74psi. I suspect the SP pressure valve in the FD has a problem, because it closes to hold RP when the FP is not running, and it regulates the SP. I would remove the SP valve on the FD and inspect it carefully. There is a rubber O-ring in it that seals pressure, and it may be damaged. I would also inquire with whoever rebuilt the FD to see if they will test and readjust it for you.

Are you familiar with the CIS Primer web site? If not, here it is:
https://cis911primer.com/home.html

[boyt911sc]

Nate,

The culprit for your hot start problem is your residual pressure loss. Both your WUR and FD are not holding residual fuel pressure long enough. The minimum is 1.1 bar after 20 mins. The problem is the way the early CIS was designed. This inherent problem was corrected in 1978 SC using a different fuel return line configuration.

Tony

[Nate W]

Originally Posted by boyt911sc

Nate,

The culprit for your hot start problem is your residual pressure loss. Both your WUR and FD are not holding residual fuel pressure long enough. The minimum is 1.1 bar after 20 mins. The problem is the way the early CIS was designed. This inherent problem was corrected in 1978 SC using a different fuel return line configuration.

Tony

Thanks for the reply Tony, it seemed to be working well for a while then started having this issue. Should I try replacing o rings in the FD supply pressure regulator? If not what if anything can I do about this? The car is so fun to drive but embarrassing when I get stranded every time I drive it far

Nate

[Nate W]

Originally Posted by PeteKz

Good info. I agree that your FD is leaking pressure, and that your FA and FP check valve are good. Yes, loss of RP will cause hard warm starting, in fact that is the usual cause.

Your SP is very low at 40psi. The SP should be between 4.5 and 5.1 bar, or 65-74psi. I suspect the SP pressure valve in the FD has a problem, because it closes to hold RP when the FP is not running, and it regulates the SP. I would remove the SP valve on the FD and inspect it carefully. There is a rubber O-ring in it that seals pressure, and it may be damaged. I would also inquire with whoever rebuilt the FD to see if they will test and readjust it for you.

Are you familiar with the CIS Primer web site? If not, here it is:
https://cis911primer.com/home.html

Thanks for the help Pete. I am now very familiar with the CIS primer page. Between that page and you guys on pelican and reading older threads I have been slowly figuring out how this system works. It’s been an interesting but sometimes frustrating process

Nate

[Zuba]

My 1973.5 cis always had a hard start when hot. Next time you’re trying to hot start, unplug the warmup injector on the back of the airbox and see if it starts better when hot.

I believe the 1974 cars had a similar setup without a thermotime switch so they spray fuel the whole time the starter is spinning.

I would unplug mine and it started easier, so now I have a momentary switch wired to it so it will only get power if I’m holding the switch.

[Nate W]

Originally Posted by Zuba

My 1973.5 cis always had a hard start when hot. Next time you’re trying to hot start, unplug the warmup injector on the back of the airbox and see if it starts better when hot.

I believe the 1974 cars had a similar setup without a thermotime switch so they spray fuel the whole time the starter is spinning.

I would unplug mine and it started easier, so now I have a momentary switch wired to it so it will only get power if I’m holding the switch.

Thanks, mine has the thermotime switch, maybe it could be malfunctioning, but I think the main issue is losing residual pressure

[Nate W]

I removed the piston in the supply (system?) pressure regulator in the Fuel Distributor but the o ring looked pretty good and felt pliable.

[PeteKz]

Hmmm. Well, the pressure is leaking down somewhere, so suspect the "system" pressure regulator until you find a different cause. While you have it apart, shim the SP valve to raise the system pressure to within specs.

[Nate W]

Thanks Pete I’ll try that. Should I just start with a small flat washer behind the spring, or is it more sensitive than that? I have seen the regulator for the later CIS models where it has the very thin shims for fine adjustment, mine just has the parts shown and a threaded cap to hold it in the FD, probably similar to your 73.5 original setup (your current engine sounds like it must be a lot of fun)

[Jay Laifman]

I'm not sure if this is covered in the discussion above. The 73.5 had a one way pressure valve that was separate. It was a little banjo fitting on the output of the fuel pump. I had one fail that caused hot start problems. I do not know how far in the 1974 that might have been there. I seem to recall the later cars had the valve built into the pump or something. Could you have a 73.5 pump without the valve attached?

[PeteKz]

Nate, just catching up to this. I've been away for a few days. IIRC, each of the thin shims in the stack provide a change of 1.5psi. Your pressure is so far out that you will need to use whatever shims or washers you can find to bring the pressure up. Make sure you have an accurate pressure gauge.

[eastbay]

This car still have the throttle valve? If so, the gauge needs to be in between the FD and the throttle valve line.