Best guess on this problem please

[calling911]

76' 911s. Permatune. New wires, new plugs.

Car intermittantly runs perfect.

Here is what happens while driving when all goes bad.

The car severely misses on any throttle application.. light, medium, heavy.. partial throttle virually stalls out the motor..

Here is how I get home when it happens:

at 30-40 mph (speed doesnt matter) I release the throttle for about 10 seconds.. then I apply the throttle and the car accelerates normally.. for about 5 seconds.. and then it stumbles.. release the throttle and nail it again (full throttle works fine).. car strongly accelerates for about 5 seconds and then falls on its butt again..

So its like its fouling the plugs? This seems odd since I just moved to NC from Colorado.. thought it might be LEAN..

Idle is fine on cold start for about 30 seconds and then the car starts to miss and goes into a cyclistic up and down deal... Im gonna go out right now and start it and after it starts to puke I'll shut it down and look at the plugs.. I bet they are wet but who knows..

Could it be a bad Permatune? I can't find any thread here where a Permatune failed in this way.. seems like they just die.. period..

Also.. I have a cap and rotor on the way.. but I just dont think that is going to be it since its intermittant.. although now it seems rather permanent..

Anyway.. limited tools for a bit until I move into my new house.. so I can't test fuel pressure but wondering if anyone has an idea on this.. or maybe a few things to try..

[HarryD]

Sounds like a mixture or vacuum issue (hunting idle) to me combined with a dirty fuel filter (loss of ommmph on acceleration i.e. fuel starvation)..

[calling911]

Originally Posted by HarryD

Sounds like a mixture or vacuum issue (hunting idle) to me combined with a dirty fuel filter (loss of ommmph on acceleration i.e. fuel starvation)..

hmmmmmmm... out to look for a leak..

[Mo_Gearhead]

I agree with Harry.

Sounds like a fuel delivery problem. Pump about to fail or fuel filter/fuel line clogged.

Rare also but: if gas tank vent becomes clogged, that can cause a vacuum in the tank and starve the pump.

Once plugs are 'fouled', they stay that way. They need to be removed and cleaned (or better yet replaced).

OOps, just saw you have new plugs!

[calling911]

Originally Posted by Mo_Gearhead

I agree with Harry.

Sounds like a fuel delivery problem. Pump about to fail or fuel filter/fuel line clogged.

Once plugs are 'fouled', they stay that way. They need to be removed and cleaned (or better yet replaced).

OOps, just saw you have new plugs!

Well, as expected, I started it and it ran great for 30 seconds and then it started to stumble. I cracked the throttle open to about 3000RPM and held it and it stumbled badly.

I then shut it down and removed the air cleaner housing (was happy to see a pop off valve) and started to feel around the back for dry hoses.. I didnt find any. I started the car again with the air filter out and it ran fine for several minutes. Put the air cleaner housing back and it again ran fine.

I then took it for about a 45 minutes drive, got some techron and added it and drove home.. The whole entire time the car ran perfectly with lots of power and a smooth idle.

So, I am thinking, when I had my arms around the engine to feel in the back the engine got enough love from me and its self esteem is now much better as a result.

In other words.. this is gonna kick my ass again and it will happen again when I least need it to happen..

I don't know what to look for at this point.. seems like it almost certainly is a electrical problem?? can one safely say its electrical at this point?

EDIT: Due to the stumble being present at 3000+ RPM when the problem was exhibited I think its a bad permatune. Anybody disagree?

[mjshira]

there are two things that 'could' be behind your issue: ignition or fuel. You need to test each system to be certain you don't have an issue. From the fuel system side, check and make sure that your fuel pump is in good working order. Since you have new plugs you can't check them to see if they are wearing evenly but you should check that if the problem continues and as you put miles on the car. The next thing is to validate fuel pressure. Could you install a fuel pressure meter and see if the problem happens while working the throttle from the engine bay?

From the ignition side it sounds like you are covering the rotor and cap, plus plugs, that is all good. But if the car performs as you said and ignition is behind it that means you've got something taking timing away from the engine, either a vac leak or bad coil maybe.

From my perspective it is fuel pump/pressure OR coil/permatune

Test those

[911pcars]

"Sounds like a fuel delivery problem. Pump about to fail or fuel filter/fuel line clogged."

+1. I'm with the "pump is bad" group, but there's an outside chance the filter is clogged.

Sherwood

[Grady Clay]

JP,

This has all the symptoms of fuel delivery restriction.

Can you find a fitting that fits the outlet of the fuel accumulator? The places to look are local shops and recyclers.

Make a hose that goes from the accumulator outlet back to the fuel fill in the left-front fender. This will let you test the fuel flow after sustained recycled flow.

The technique is identical for MFI in CMA2. You flow fuel back to the fuel tank. Measure the initial 30 seconds of continuous flow and again after five minutes and again much longer. If you hear the pump change tone, quit.

Your ’76 CIS is the first year with the electric fuel pump back at the front of the 911 (this is good after a hiatus since ’70). This tests the flow through the fuel tank filter screen, the electric pump, the fuel filter and the accumulator in addition to many pipes and hoses. To override the switch, you will have to raise the sensor plate manually or invent an electrical bypass.

The flow specifications are; minimum 750 ml/30 seconds with at least 11.5 V at the fuel pump.


I suspect you may find the filter screen in the bottom of the fuel tank restricted. I won’t discount any other possible cause.


All of my prior admonitions about fuel safety apply. Gasoline is the most dangerous substance we normally ever come in contact with. BE CAREFUL!

Best,
Grady

[3.2 CAB]

calling911, is the car still doing OK? If so, there was something you said that made me think of what a possible problem could have been. You stated: (was happy to see a pop off valve) does anyone else think that the pop-off valve might have not been seated correctly? The way he was describing the faults, sounded very much like a vacuum problem, then when you said that about the pop-off valve, it all seemed to make it a possible problem, and when you removed the air cleaner, the pop-off valve got re-seated, and then it ran great. Anybody else have this thought??

[calling911]

Originally Posted by 3.2 CAB

calling911, is the car still doing OK? If so, there was something you said that made me think of what a possible problem could have been. You stated: (was happy to see a pop off valve) does anyone else think that the pop-off valve might have not been seated correctly? The way he was describing the faults, sounded very much like a vacuum problem, then when you said that about the pop-off valve, it all seemed to make it a possible problem, and when you removed the air cleaner, the pop-off valve got re-seated, and then it ran great. Anybody else have this thought??

Pop off valve is not in the intake leak category.. but good thought. When I took out the filter the first time the car was running and running crappy.. and it continnued to run crappy.. I didnt mention that in my previous post.

Grady; I am not sure why I'd have an intermittant problem if the screen were clogged but I wont question you on it.. The fuel pump does run very nicely with the key on.. no intermittant nature to it and it sounds healthy.. but that means very little I guess if it doesnt flow all the time.. I dunno.. I have never heard of an intermittant fuel pump.. but I wont discard it as a possibility either.

Hmm it sounds like its unexpected that my pump would run with the ignition on.. wonder what that means?

I think now that its cooled off again I shall go out and take it for a drive again.. I'll report back....

[Grady Clay]

JP,

The ‘restricted intake screen’ symptom is that the engine runs well at first and then ‘dies’ after full throttle or extended highway use. The reason is the circulating fuel flow picks up debris from the bottom of the fuel tank and the flow attaches it to the screen filter, restricting the flow. Once the engine is off (particularly if it is being towed home), the debris falls off the screen and reasonable flow resumes.

This is why it is intermittent.

This is a common problem with pre-CIS 911s (usually MFI but carbs also).

Best,
Grady

[Walko]

could be that if it was a vacum leak when you had your hand at the back of the motor giving it a special cuddle you bumped the leaking hose or ....

just my two cents worth

Michael

[David E. Clark]

Based on your description of the problem, I would agree that this is a fuel issue. I would check for problems in this order.

1. Clogged fuel filter or screen as Grady suggested.
2. Vacuum leak(s).
3. Faulty or clogged fuel injector.
4. Faulty fuel pump or low fuel pressure (it should be at least 66psi).

It may also be a fault in the ignition system. In your original post you said that a distributor cap and rotor were on the way but if you haven't changed them yet, check for crack(s) in the distributor cap. This can cause your symptoms. You should also check the condition of the rotor and if you are still running points make sure the gap is correct (.016). Finally, if your ignition timing is way off you might experience similar problems but, based on your description, it's unlikely that this is your problem. Good luck and keep us informed.

David

[3.2 CAB]

Hi JP, I was just taking a chance on it being the pop-off valve not seated, because of a lot of other postings that I had read about how crappy the engine runs, or even not starting at all due to this condition. I personally can say that I have never even touched one of them, being I have a 3.2. The way you described your problem sounded a lot like when I rebuilt my injectors, and knocked the large hose loose on the "Y" shaped tee, on the drivers side almost against the firewall, when I was buttoning it all back together. It was on the large cross-over pipe that runs from the right side to the left side, and meets at that "Y" tee. The large hose end was cracked and aged, so I trimmed it off to a good end, then put hose clamps on every hose/tube, that connects to the "Y" shaped tee. O-well, it was a shot at it. Hope you get it worked out soon, good luck, Tony.

[calling911]

Originally Posted by 3.2 CAB

Hi JP, I was just taking a chance on it being the pop-off valve not seated, because of a lot of other postings that I had read about how crappy the engine runs, or even not starting at all due to this condition. I personally can say that I have never even touched one of them, being I have a 3.2. The way you described your problem sounded a lot like when I rebuilt my injectors, and knocked the large hose loose on the "Y" shaped tee, on the drivers side almost against the firewall, when I was buttoning it all back together. It was on the large cross-over pipe that runs from the right side to the left side, and meets at that "Y" tee. The large hose end was cracked and aged, so I trimmed it off to a good end, then put hose clamps on every hose/tube, that connects to the "Y" shaped tee. O-well, it was a shot at it. Hope you get it worked out soon, good luck, Tony.

No problem.. just so you know the pop off is pre the throttle plate so its not under vacum,


I just got back from another run.. its a cool foggy night here.. car ran very well..

Grady; I hear what you are saying but remember, this was also at idle.. if it were just full throttle or even part throttle I would be all over the fuel system.. it ran great for 30 seconds on a cold start and then bad misses..

I doubt a clogged anything would clog and unclog intermittantly don't you??? And especially in my scenerio where the car was sitting and went from bad to good without even being moved...

and as I mentioned.. for some reason my car is wired so the pump is on when the key is in the run position... and it sounds very smooth and consistent.. however.. Also, I definately noticed the pump sounded fine when the car was running bad at idle (it died many times while playing with vacum lines and when it dies the pump continues to run)...

So.... it happens at idle, part throttle, full throttle and its intermittant... and of course now its working fine...

I suppose it could be the fuel filter... I mean, if you recall, when it would happen if I let off the accelerator and let it coast and then nailed it it would light up and run for 5 seconds or so.. so maybe its because the fuel pressure built up enough with the throttle closed to give it 5 seconds worth of run time...

So I guess I will replace the filter for sure.. why not? It looks old anyway.. and then if happens again I will have to follow Grady's advice and try and check flow...

Hmmm now that I think about it.. the accelerator scenario I just mentioned sort of rules out the Permatune.. I mean why would the Permatune start to work after coasting? Right?

Hmmmm the whole coasting then apply the throttle and it goes scenario is a giant clue here..

EDIT: David a vacuum leak would not be an issue at full throttle I dont think.. and this happened at full throttle. (am I correct here?) I suppose I could have a bad injector.. but intermittant?? Doesn't sound right..

[David E. Clark]

JP

Yes, as unlikely as it sounds, vacuum leaks can be intermittent. Sometimes heat expands a small hole in one of the hoses or one of the hose clamps contracts and expands. You also see this in old gaskets which are sometimes "sealed" with oil and sometimes not. You can even get vacuum issues if the oil cap gasket is old so that the cap is not firmly seated (this is unlikely to be your problem!).

The more I think about your problem I wonder if there is a simpler solution. Is there is any chance this could be as simple as corrosion on the electrical connections of the fuel pump relay or perhaps the relay is not firmly seated? This could cause intermittent fuel pump operation and thus intermittent running. Just a thought.

[911quest]

Your signature said the car awoke from hibernation is this correct if so was the tank cleaned? What could be happing is if the fuel level is low and after driving for awhile and some turns etc the pick-up can get clogged. That might also be why it will some time run ok while it is sitting cause all the crap is the tank has settled back down to the bottom......just a thought

[911quest]

Just re-read the post Grady posted the same

[calling911]

Originally Posted by 911quest

Your signature said the car awoke from hibernation is this correct if so was the tank cleaned? What could be happing is if the fuel level is low and after driving for awhile and some turns etc the pick-up can get clogged. That might also be why it will some time run ok while it is sitting cause all the crap is the tank has settled back down to the bottom......just a thought

Yeah and I dont know what I can do now to troubleshoot it now that its running fine. If I check pump flow it will undoubtedly be fine.. but I will change out the filter... There is no way to get to the tank screen is there????

EDIT: and yes, car was just taken out of storage a month ago.

[calling911]

Well, after sitting for about 36 hrs I started it this morning and it did the exact same thing.. idles fine for about 30 seconds and then it started to sound like my lawn mower.

Seems like the only theory that makes sense is that I have stuff in my fuel tank.. crap.