Intermittent No Start, how to test?

[acorad]

'81 911SC, I've been dealing with this problem for months so I appreciate your help!

How do I do a fuel pressure test on an intermittent no-start problem?

Intermittently, like every 6 or 7 starts, the car cranks but doesn't start. During the no-starts the fuel pump has power and is running, but there is no gas smell from the exhaust pipe, so my conclusion is that the injectors are not running and therefor it's a CIS issue.

When this happens I can always get the engine to start by first lifting the air sensor plate in the airbox such that the fuel injectors run for a couple seconds, and after that when I crank the engine it starts (it always starts on the 2nd crank, if that matters).

I'm not super clear on how exactly to use a fuel pressure gauge to test the CIS, but I think I need to test the CIS during a no-start episode, right? But as the problem is intermittent, how do I do that?

Can I attach the gauge to the CIS and drive around with it on the engine? And when a no-start episode does happen, then I can read the gauge?

[boyt911sc]

Originally Posted by acorad

'81 911SC, I've been dealing with this problem for months so I appreciate your help!

How do I do a fuel pressure test on an intermittent no-start problem?

Intermittently, like every 6 or 7 starts, the car cranks but doesn't start. During the no-starts the fuel pump has power and is running, but there is no gas smell from the exhaust pipe, so my conclusion is that the injectors are not running and therefor it's a CIS issue.

When this happens I can always get the engine to start by first lifting the air sensor plate in the airbox such that the fuel injectors run for a couple seconds, and after that when I crank the engine it starts (it always starts on the 2nd crank, if that matters).

I'm not super clear on how exactly to use a fuel pressure gauge to test the CIS, but I think I need to test the CIS during a no-start episode, right? But as the problem is intermittent, how do I do that?

Can I attach the gauge to the CIS and drive around with it on the engine? And when a no-start episode does happen, then I can read the gauge?

You can test your fuel pressures without running the engine. All you need is to test run the FP and use a pressure gauge. For a CIS motor, you need to bridge terminals #87A & #30 of the FP relay socket (FP relay removed) and ignition switch @ RUN position (not start).

Or disconnect the air flow meter sensor switch plug and ignition switch @ RUN position and the fuel pump will run. This is CIS 101. This is the basic test any CIS troubleshooter should know or understand to become successful in diagnosing the problem/s.

Tony

[acorad]

Thanks Tony, but I must be missing something. The problem is intermittent. Shouldn't the fuel pressure be tested when the problem is actually occuring?

[Schulisco]

Intermittent issues are boss enemy.
I would first determine if it's fuel or ignition related ... carry a can of starting fluid you can apply when it again won't start ...

Originally Posted by acorad

Can I attach the gauge to the CIS and drive around with it on the engine? And when a no-start episode does happen, then I can read the gauge?

Yes, no problem. Not for ever of course, but a quick test drive shouldn't matter.

Thomas

[boyt911sc]

Originally Posted by acorad

Thanks Tony, but I must be missing something. The problem is intermittent. Shouldn't the fuel pressure be tested when the problem is actually occuring?

Yes. You are missing the basic knowledge how to test a CIS. You are guessing instead of investigating and testing your fuel pressures. You need to check your basic parameters for fuel pressures, ignition, and air leaks. Test and verify. Do you have a CIS pressure tester?

Tony

[acorad]

Originally Posted by Schulisco

Intermittent issues are boss enemy.
I would first determine if it's fuel or ignition related ... carry a can of starting fluid you can apply when it again won't start ...

Thanks Thomas, I was assuming it was fuel related as there is no gas smell out of the exhaust pipe during the no-start episodes.

Yes, no problem. Not for ever of course, but a quick test drive shouldn't matter.

Thomas

Thanks, good to know.

[Schulisco]

Reread your start post, got some questions:
1. Is it possible that the issue is ambient temperature related?
2. How many turns do you need to fully close the idle rpm screw? (reset aftwards!)
3. Does the fuel pump runs when switching ignition on or does it only run when engine is cranked?
4. Did you checked already the mixture on a certified gas tester? ('81 SC US model with 930/16 engine the CO value must be 0.4-0.8% on warm idle!)

Thomas

[acorad]

Originally Posted by boyt911sc

Yes. You are missing the basic knowledge how to test a CIS. You are guessing instead of investigating and testing your fuel pressures. You need to check your basic parameters for fuel pressures, ignition, and air leaks. Test and verify. Do you have a CIS pressure tester?

Tony

Thanks Tony, yes, I am absolutely missing the basic knowledge of how to test a CIS. I think I said that in my original post. No, I do not have a pressure tester, and, yes, I am perfectly happy to get one and learn.

Regarding air leaks, I ran the car and sprayed carb cleaner all around the engine, airbox, etc., and I did not see any change in rpm. I ran the engine and pulled the cap off the oil fill tube, and did not see any change in rpm. So I assume it's not air leaks.

Regarding ignition, there is no raw gas smell out of the exhaust pipe during the no-start episodes, so I assume it's not ignition.

How about a new question: What should be tested when the problem is actually occuring?

[Schulisco]

Originally Posted by acorad

How about a new question: What should be tested when the problem is actually occuring?

Carry an old spark plug with you, when the issue occurs - put it in one of the spark cables and make it connected with the engine case for ground contact (additional wire!) . Danger: High (fatal) voltages!!!!
Then let the engine crank by another person and look for a spark...don't let the spark plug being unconnected to ground while cranking because a single shot can kill your CDI ignition module!!!

Thomas

[boomvang1]

You say you ran the engine and pulled the oil cap with no change in RPM? I believe that means a vacuum leak.

[917_Langheck]

Originally Posted by acorad

I ran the engine and pulled the cap off the oil fill tube, and did not see any change in rpm. So I assume it's not air leaks.

Regarding ignition, there is no raw gas smell out of the exhaust pipe during the no-start episodes, so I assume it's not ignition.

Assumptions are the antithesis of testing. Never assume, always test.

You have an open to atmosphere condition if the idle doesn't change after removing the oil cap.

[acorad]

Thanks Thomas, as I think about this I'm pretty sure I have an inductive [edit] timing light around somewhere from my old 240Z days. I will try to find it.

Also, I will redo the oil cap test. I did it months ago and may not be remembering the results correctly.

Also,

1. The issue does not seem to follow ambient temps, it seems the number of starts, like every 6 or 7 starts it happens no matter what.
2. I'll check that
3. I do not hear the FP run when the engine is off and the key is in the "run" position, if that's what you're asking. I ran some wires from the FP to the cabin and hooked them to a multimeter and saw that the FP is getting power when cranking during the no-start events.
4. No, first time I've heard of that.

[acorad]

boomvang1 and 917-Langheck, thank you, I may be mixed up on the oil cap test as I did it months ago, I'll redo it.

[PeteKz]

accord: The fact that you can lift the air metering plate, hear the injectors squirting fuel, and then it starts, tells me that your are not lacking spark, only fuel. While it's a good idea to check for spark, focus your efforts on finding why you lose fuel. Squirting starter fluid into the intake should accomplish the same thing--giving it enough fuel to start.

Now, my guess is that this is more of a problem when the engine is warm, rather than when it is cold. Is that the case?

Tony, Thomas, others: I'm thinking possible loss of RP.

[acorad]

Hi Pete, thanks, it happens whether the engine is warm or cold. Every approximately 6 - 7 starts.

[boyt911sc]

Originally Posted by PeteKz

accord: The fact that you can lift the air metering plate, hear the injectors squirting fuel, and then it starts, tells me that your are not lacking spark, only fuel. While it's a good idea to check for spark, focus your efforts on finding why you lose fuel. Squirting starter fluid into the intake should accomplish the same thing--giving it enough fuel to start.

Now, my guess is that this is more of a problem when the engine is warm, rather than when it is cold. Is that the case?

Tony, Thomas, others: I'm thinking possible loss of RP.

Peter,

You need to test and eliminate the most likely culprit/s. Your guess is as good as anybody’s guess. For ignition, I use an inductive timing light. For fuel pressure, I use a pressure tester, for air leaks I use a smoke generator. For presence of 12-volt, I use an automotive test light, etc.

Without doing the appropriate tests, it would be very difficult to identify the culprit/s in CIS. The less variables in an equation, the easier to solve the problem. The fuel pump might be failing, so test it. The CSV might not be working, so test it.

If you believe it is residual pressure loss, then test it.

Tony

[acorad]

Just drove the car and did the oil cap test, taking the oil cap off dropped the idle rpms by about 100.

[PeteKz]

No Tony. My guess, and your guess too, is not as good as anybody's guess. It's better. You and Thomas, and several others here have been working with CIS and solving problems with it for longer than the great majority of other people. That gives insight and understanding that helps us narrow down the scope to the most likely problems, to efficiently focus troubleshooting there first.

I used the word "guessing" not literally, like picking lottery numbers, but to tentatively indicate that I am considering a particular hypothesis. It's a figure of speech.

Based on the information acorad has provided so far, lack of ignition is very unlikely. So we should concentrate on fuel until something suggests we should go back to ignition. The squirt of starter fluid when the car won't start will prove dispositive.

On the fuel side, we can be "pretty sure" that the SP is not the issue because the car runs normally after it starts. Same for the WCP. I also don't think the CCP is responsible, because the car starts normally from cold most of the time. Instead, we should focus on the starting part of CIS operation. As we well know, a common starting problem is a warm restart, due to loss of residual pressure. That's why I asked--in a figurative way--whether this was a problem during warm starts or cold starts. He answered that it happens either way, so we can surmise that loss of RP probably is not the cause either.

Thus far we have not needed any meters, gauges, or even test lights, but we have narrowed down the scope of potential problems, and thereby focused the troubleshooting he has to do. It's easy to say that he needs a CIS gauge to run tests, but he doesn't have one (yet--acorad, take the hint). But, one can do plenty of troubleshooting on CIS without that gauge, so let's help him do that.

Acorad: Back to you. Order the fuel injection test set, so that Amazon can drop it at your door by the weekend. Even if you get the problem figured out sooner, you will need it eventually. This is what I have:

https://www.amazon.com/BETOOLL-Injection-Pressure-Tester-Gauge/dp/B01L17TPT2/ref=sr_1_5?crid=1SI65R8F69XHP&dib=eyJ2IjoiMSJ9.RhI0alR9cPOvMk8oUSKHwcabbZ2HUFnLc nswP5m1-9o6TQV2AZHKuq5hKgOnxpFCjCs0F2ySYF87r8I1IYFp8PMGI_h kkhn8ffhd_0hI2wb68s56garOvtmEKftQu6tZpONvdsI7P3DeU qjBUtTwlpWyPiY-mRs4H3XFHWhIHYWzKgOjzxnkaGuAZjhnof8L5yQ6OVIylqdOsC dS1q_r0krTKjd7GJj9K4RgKBDrfsMfRaV1tlJljIe_7KfUzjwO MpthA9n1kClhHfpVly8XIpy5mRrDzc95JrlMyJuIpKM.zejwtf 08szoNAHqlKDt8YU0AVgPiIaGTpULTjarD83A&dib_tag=se&keywords=fuel%2Binjection%2Btest%2Bkit&qid=1727838536&sprefix=fuel%2Binjection%2Btest%2Bkit%2Caps%2C174&sr=8-5&th=1

Next, intermittent problems most frequently are electrical. CIS has two electrical circuits. The first powers the cold start components (WUR, AAR, Thermo Valve and Thermo Time Switch, and Cold Start Valve). The second powers the fuel pump, and includes the safety switch on the front (towards the front of the car) of the air metering unit. Because the engine starts from cold most of the time, the cold starting part of the system most likely is working. That leaves the fuel pump.

I suggest you do this: Unplug the connector to the safety switch. This will make the pump run when you turn the ignition ON (otherwise it doesn't run until you turn the key to START). This will allow you to hear whether the pump is running soon as you turn the switch ON. Then you can listen to see if it runs while the problem is occurring.

Let us know what you find over the next few days. And order the kit. And do the starter fluid test.

[PeteKz]

Acorad, re your oil cap test, that's good. It does not mean that you have NO vacuum leaks, but it does mean that if there are vacuum leaks, they are not significant enough to cause your starting problem. You can return your attention to troubleshooting fuel.

[acorad]

Pete, thank you, will do. I should be able to disconnect the safety switch tomorrow and the fuel injection test set should arrive on Friday.

Question, does the safety switch connector have a specific method to release the connector? Or does it just pop out when you pull on it?