Hot starting rough idle if lucky.

quattrorunner · quattrorunner's Garage

Strange title I know.
79 911sc stock ignition.
I have an issue I think with the ignition. In the morning, the car starts normal and runs normal. It will restart within 5 minutes normally, but after like 10 minutes when it's at opperating temp all the way till the rest of the day sitting in the hot sunny las vegas heat(100-103 so far) it will really run rough like it's missing a couple cylinders or somthing. It sometimes takes some time to start. When it does start, it will get progressivly better within 10-15 seconds till it returns to normal and then is fine.
I checked the plugs and swapped wires. My cap and rotor are not old but the cap has an irregularity on two points. Like it is gouged there. Any ideas on if it's just a cap or something more?
Thanks

Paulporsche · 06-02-2007, 11:19 AM

If the rotor looks OK, check the lateral movement in the distributor shaft. That out of round spin could be causing the gouging.

quattrorunner · quattrorunner's Garage

Does that require a dist rebuild?

Paulporsche · 06-02-2007, 12:14 PM

I think it would. But before that, check to see if there is any side to side play in the shaft by holding it @ the top (or @ the rotor). Do you get any movement?

stlrj · 06-02-2007, 03:08 PM

forget your cap and rotor. If that were your problem, your symptoms would be present all the time.

It sounds more like a lack of residual pressure problem due to a bad fuel pump check valve.

Can you feel any resistance when you push up on the sensor plate when the engine is warm?

Joe

quattrorunner · quattrorunner's Garage

Hmm I'll check.

Miguel Antonett · 06-02-2007, 06:40 PM

It could be airlock, due to the heat and gas vapors.
Use all the gas that is in the tank, fill it up with fresh gas, add some extra Techron and go for a good ride - open it up - to "flush" the system.

quattrorunner · quattrorunner's Garage

It really seems electrical. It has an on off feel to it sometimes. Like tonight, it was doing the same symptoms when it just clicked to normal as thoughthe switch was thrown.
The shaft does have some play, but when I hold it to the top and move it back and fourth, it's not too much, but the cap does have some scoring on the plastic wall

Car runs well otherwise.

Hoffmanmotors · 11:18 PM

I have this same exact problem on my 79sc. I was told by many that it sounds like the fuel pump check valve or fuel accumulator. I replaced the accumulator but it didnt fix the problem, I have ordered a fuel pump check valve from our host. I'll let ya know if it was the culprit.

quattrorunner · quattrorunner's Garage

Thanks

schoward · 06-03-2007, 06:34 AM

I had same experience - and like Hoffman tried Fuel Accumulator. No luck. Changed fuel pump check valve and never an issue since.

One thing helped me narrow this down somewhat. IF your problem is related to lack of residual fuel pressure, I believe what happens is the fuel in your lines (especially the ones that run across the top of your engine), basically boils after hot shut down due to the intense heat and lack of pressure in the lines which results in vapor lock.

I noticed that if I left my deck lid open after hot shutdown (thus letting cool air in help cool off those hot fuel lines and reduce the boiling), the problem was dramatically reduced.

Paulporsche · 06-03-2007, 07:57 AM

Agreed re accumulator and check valve advice, but what's causing the gouging in the cap and cap contacts?

Can you swap in another rotor? Does a close look reveal any defects?

boyt911sc · boyt911sc's Garage

Quattro,

Check your residual, control, and system pressures before you start replacing parts. Why would you replace CIS parts if they are not confirmed defective? Test and verify that the suspected part is malfunctioning or defective. You need a CIS fuel pressure gauge kit to do this test.

Tony

stlrj · 06-03-2007, 12:46 PM

Quattro,

I know you don't have the CIS fuel pressure gauge, that's why I suggested the simple residual pressure test of pushing on the sensor plate to see if you feel resistance which means residual pressure is present if resistance is felt.

Saves time and money...

Joe

quattrorunner · quattrorunner's Garage

You guys, I have been busy and will try the residual pressure in the meter, but I honestly don't hold much stock in it, it just doesn't feel like fuel issue to me. I promise to test the resistance in the meter tomorrow and report.
Tonight 7:00 p.m. The car had some of the symptoms after not running at all today. The garage is hot from another car and the sun on it in the afternoon. So now why does heat affect the starting of the car? Odd.

SCWDP911 · SCWDP911's Garage

What did your problem end up being quattro? I had a similar occurrence today (for the first time ever) on my way home from NW Arkansas (Porsche Palooza). Nice day out, 70's, but there was some very spirited driving on some very nice roads. Got to first gas stop, filled up, started car, and it ran very lopey at a low idle (about 300-500 rpm). It would die if I did not help it with some gas. But then, half a mile down the road, it was fine, and would stay fine, even when coming back to an idle condition, until I shut down engine and tried to restart.

quattrorunner · quattrorunner's Garage

Well, I replaced the accumulator. That seemed to make it better somewhat. But then it got cooler out thus almost eliminating the condition. Now I believe the pump went out the other day. Will not even run.(this is months since the other issue Sorry) I assume it's the pump since the fuse is in tact and it tries to light but it just will not run. I haven't gotten the chance yet to get into it.I assume the checkvalve comes on this particular pump and that will fix two issues at once. Any thoughts on this?

quattrorunner · quattrorunner's Garage

Just another update. What I thought was a bad fuel pump turned out to be that my pop open valve in the air box got blown out somehow. I didn't hear anything odd, it just didn't start. And I could not hear the pump cause the buzzer is so loud I just figured it was the pump. Anyway, I got it towed home and within three minutes it was running again. Odd but good news. So I still have an old pump and check valve But it runs fine.

quattrorunner · quattrorunner's Garage

Just another update. What I thought was a bad fuel pump turned out to be that my pop open valve in the air box got blown out somehow. I didn't hear anything odd, it just didn't start. And I could not hear the pump cause the buzzer is so loud I just figured it was the pump. Anyway, I got it towed home and within three minutes it was running again. Odd but good news. So I still have an old pump and check valve But it runs fine.

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by stlrj

Quattro,

I know you don't have the CIS fuel pressure gauge, that's why I suggested the simple residual pressure test of pushing on the sensor plate to see if you feel resistance which means residual pressure is present if resistance is felt.

Saves time and money...

Joe

Quattro,

Basing from your experience, do you think you save time and money by replacing CIS components at random? Or would you say doing quess work to troubleshoot CIS problem/s? Please don't get me wrong, the reason I made this post is call the awareness of people planning to do their CIS troubleshooting in the future.

You are not alone and I was in similar situation many years ago before taking a serious approach to learn the basic CIS principle. If anyone is planning to do the CIS troubleshooting, please use a CIS fuel pressure gauge kit. Know your control, system, and residual fuel pressures and you could almost analize correctly what's going on in your injection system. Just my two-cents.

Joe,

Sorry for quoting you. There's no intention to malign your post because your intention was to help Quattro and deserve appreciation. Please accept my apology if I have offended thee.

Tony