83 911 SC engine quits when warm
 

I have a 3.0 with 155,000. Good shape but has started a bad habit. Cold it starts quick but sputters until warm. When warm it idles with occasional misfire. After 30 minutes of driving or idling it begins to sputter, backfire and quits. Starts up with little issue and then runs rough for a few minutes. Then runs fine until the next bout with sputter, loose power, backfire, and quits. Fuel full, oil and temp in normal ranges, checked as many vacuum lines (as I can find). Any ideas as where to start my search? Thanks in advance.:(

Could be incorrect control pressure. Do you have fuel pressure gauges?

Could be air box leak. Do you have a pop-off valve installed?

Control plunger could be sticking or binding.

O2 relay could've gone bad. If it started all of a sudden that would be my guess.

Could also be a few other things like leaking injectors but we need to start with the basics.

Quick test for vac leaks. When the engine is warm (between bouts), remove the oil filler tube cap. If the idle changes, that is a good sign. If the idle doesn't change that means you have a significant vac leak. If you'd had a backfire, it could mean the airbox has a crack.

sounds like it is running lean.

tighten all vacuum type connections

3.0 quits when warm
 

Have pop off valve, plunger feels free, moving easily, engine idle changes when removing oil filler cap. I do not have pressure gauges, where can I find those? I will look up how to check O2 relay. Not finding any fuel leaks. Any advice?

I would check the fuel supply. Could be a good idea(and cheap) to replace the fuel filter and perhaps cut it open to see if you can find traces of water and debris of some kind. I had a friend who had this kind of problems that kept coming back. He had a very small hole in the fuel filler line where dust and water came in and made the engine run like crap. Very difficult to find. I guess a cold engine will have less problem due to more fuel being injected making the problem less obvious

would check the Warm Up Regulator,
In my car that was the problem for warm idle also

You can get the fuel pressure gauges from Pelican or Amazon. You can go ahead and order the O2 relay. If you don't need it now, you eventually will. Good insurance to have. It is a little cube under the passenger seat.

When I bought my '83 a couple of years ago I had several issues. Weak spark - changed plugs, plug wires, rotor and cap. Changed fuel filter. Ran a can of BG44k through the fuel. Also the cold control pressure was too low. It could be a few things, so let's be methodical. Tony (boyt911sc) and some of the other experts will chime in too.

There are a few things tests that can be done in the meantime before you start swapping parts.

Some things to check and report back:

- With the key ON but the engine not started, take off the air box cover. You should NOT hear the fuel pump running. Reach in and push up on the pressure plate arm for 1 or 2 sec, but no longer. You should hear a distinct buzz. That is the frequency valve. Report back.

- You can't really check the WUR without the gauges but you can check the resistance of the bimetallic strip using a multimeter. You should have a type -090 on an '83. Pull the electrical connection off the WUR and use a pair of alligator clips to connect to the metal spades on the WUR side. The resistance should be around 25-26 Ohms.

- Unplug the O2 sensor and see if it gets better. Be advised the O2 sensor connection is really brittle and it could break completely when you mess with it. It's located underneath the fuel filter on the left side of the engine compartment.

- Fuel delivery could be a problem. Could be fuel pump, check valve or leaky injectors. Hard to diagnose unless you have gauges. You can do the injector test by pulling the injectors and putting them in something like baby food jars (just small clear containers). You can fire them by pushing up on the pressure plate arm. They should not be leaking and should spray a fine mist.

Might also be a leaking cold start valve. That would make the engine run excessively rich and it will likely idle rough when cold or warm. The rough idle would likely get worse as the engine warms up, but that doesn't sound like what yours is doing.

when he lifts the plate he will hear the fuel pump will he not regardless of if the frequency valve is working ..

I would also have a look at the plugs and note their condition .. as far as the pop off valve goes they are more trouble than they are worth, make sure it is not leaking past the epoxy and the lid closes and seals securely.


Also when you say backfiring do you mean through the intake or the exhuast ?

Right. He should hear the fuel pump run and the FV when he pushes up on the arm. My pressure plate sensor was disconnected so I would hear the fuel pump running constantly with the key in the ON position, which is not normal.

Good point on the pop-off valve. If it is not closing properly or the epoxy around it is cracked it could certainly be sucking false air. They are cheap insurance against a blown air box but that's all. When I first bought my car I would hear it pop on a regular basis. It was as if that's how the car is supposed to run. Lot of myths out there...

Reporting back on fuel pump check
 

OK key on, could hear high pitch whine, lifted pressure plate and heard the fuel pump kick fuel into the system. Ordered an O2 relay and new fuel filter. Have to wait until evening to run some of the other checks. Those little fuel pressure gauge sets are not a cheap tool, I am going to check with a few friends to see if anyone has one I can borrow first.
Thanks, Mike

No they are not cheap. :( But they are a must-have tool for working on these cars. If you put your location in your profile or post it, someone on here might be willing to lend you a set.

When you say high-pitched whine, let's make sure we're talking about the right thing.

When you push up on the pressure plate there should be a distinct buzzing sound coming from in front of you. If you did not hear that buzzing it means the FV is not working as it should.

If you hear more of a whirring sound (I'm not sure how else to describe it). That is the sound of the fuel pump. You should not hear it running with the key in the ON position. If you have skinny enough arms, take your left arm and reach in and try to feel around on the backside of the engine for a loose wire. The connector will be a blueish green color if you can see it with a mirror and flashlight. That is the pressure plate sensor and it should be plugged you. You may have to fumble around blind but you will feel where it plugs in. You should definitely have it plugged in for safety's sake. It is designed to cut off the fuel pump in certain accident situations.

People usually describe the CDI as being a high-pitched whine. I have a Permatune instead of the original Bosch. The Permatune CDI doesn't make that sound so I can't tell you what to listen for there.

I forgot to ask an important question! Is your car a US or RoW model?

CIS troubleshooting..........
 

When you lift the AFS (airflow sensor) plate with the ignition @ ON position, the whirring noise/sound is NOT THE FP!!!!!!! The FP is located at front of the car unless you have a bad FP to make that loud noise. The noise is generated by the 6 fuel injectors spraying fuel simultaneously. Go back to the car and try it yourself.

Tony

Poor choice of words on my part. I was just trying to highlight that the FV is a distinct buzzing sound and there is a whirring sound when the FP is running.

It is US model. I ordered new fuel filter and O2 relay, checked the air box and pop off. I did find some questionable vacuum lines. Went to parts store to pick some up. Just as I pulled in the car lost power, began missing, exhaust smelled very rich. Came back out of store car started right up and repeated the missing and rich smell about 10 minutes later. Parked it at the office, try again when I go home. Ordered parts will be in on Friday so I will get those swapped in and see what happens from there.

The spray sound I hear, after a second or two I hear the fuel pump (?) kick in. I believe the high pitch whine is the Bosh CDI. Comes on as soon as I turn on the key. The one thing I keep coming back to is the car runs great during warm up. 10 to 20 minutes later it starts the rough running dance. Cool down while at lunch, starts and runs great. I am currently doomed to 10 minute road trips :(

Parts received and replaced. Partial success but still quits.
 

Received fuel filter and O2 sensor relay and installed them. Replaced some 4mm vacuum lines while I had it apart.
Items checked are vacuum lines and connections, remove oil filler when running, engine reacted, pop off valve o ring in place and valve working smoothly, air flow sensor plate centered, operates smoothly, closes flat. Pulled and cleaned all contacts on ignition relays and connections just for fun. Last tune up was 5/2012 with about 1000 miles on the tune up.
Started it up, ran smooth went right to 900 rpm. That is the first it ran that well cold. Let it sit and idle. At the 20 minute point (engine now warm) it began to miss, then sputter, then at 21 minutes die. Looks like you guys solved one problem (rough running when cold) but I need to figure out where to go next. I like the methodological approach but could use some thoughts as to where to go next. I am taking it in for a valve adjustment by a Porsche mechanic in a couple of weeks at his suggestion. I would do it my self if I was more confident in what I am doing, but kind of saving him for my back up plan as I would like to learn this box engine myself. This appears to my uneducated mind to be heat related. Engine runs great until it gets to temperature then the dance starts. Cools down for a few minutes and away we go again until the temp (I presume) comes back up to the shut off the power temperature. Any path to follow would be great. :confused:

Micheal,

Fuel pressure test is going to tell a lot. Too high or too low control pressure will make the air-fuel ratio incorrect, causing a rough idle. Here is a write-up I did on how to do the test and other troubleshooting.

http://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies.html

It is a work in progress.

Glad we got one thing resolved. Let's get the next one! :)

What is your engine temp reading when it starts stumbling?

One thing is that the injectors or the cold-start valve (injector) could be leaking. But it seems odd that it only starts after 20 min so I'm inclined to think that is not the issue.

The ECU under the passenger seat could be suspect. It is responsible for adjusting the air-fuel mix between idle and WOT.

I'm wondering if your CDI box might be in the process of dying. I don't have a Bosch so it's hard for me to say. Maybe some others could chime in. If it is the CDI, you have 2 choices: get it rebuilt or do the MSD upgrade. Use the search function and you should find plenty of threads on both. When it starts to stumble, you might try testing the spark. You can use a timing light and move the inductive connection from plug to plug to see if the light is firing. Double check the timing while you're in there. If you're not getting good spark, it could be plugs, plug wires, distributor cap, rotor, or coil. But let's not get too far ahead of ourselves.