Engine is fat when restarted

[1963SWC]

Maybe one of you has had this issue and can help me identify the piece of equipment that is causing this.

Start the car when cold and the car runs and acts normal and runs great as long as you keep it running. If you never cut the ignition off you can drive the car for 30 minutes or longer with normal operation. If you kill the car and immediately restart the car, the car starts but is very fat, 200 rpms slower than normal and you can small the unburned fuel and if you attempt to drive it, very poor throttle response and it pops constantly.

I have tried to cut off and back on again very fast multiple times and the condition still exist. Let the car cool down completely and the first start you are good to go, until you cut the car off and restart, back to the same fat condition and popping.

Bad O2 sensor? WUR? AAR doing something crazy? It has to be something electronic that re-measures after it warms up and changes setting, I guess.

[Bucketlist]

First thought is WUR voltage, and AAR voltage I guess, make sure 12v is present.
Do you have an AFR gauge and/or Control Pressure gauge in the car to keep an eye on?

[flightlead404]

I'm pretty sure I've seen this behavior myself. For the life of me I can't remember what it was.

I think I would start by disconnecting and cleaning all electrical connections especially ground and examine the pins on the connectors and relays.

[908/930]

Long shot, check if your cold start injector is leaking, or just unplug it and see if there is any change.

[BauerR]

Originally Posted by 1963SWC

Maybe one of you has had this issue and can help me identify the piece of equipment that is causing this.

Start the car when cold and the car runs and acts normal and runs great as long as you keep it running. If you never cut the ignition off you can drive the car for 30 minutes or longer with normal operation. If you kill the car and immediately restart the car, the car starts but is very fat, 200 rpms slower than normal and you can small the unburned fuel and if you attempt to drive it, very poor throttle response and it pops constantly.

I have tried to cut off and back on again very fast multiple times and the condition still exist. Let the car cool down completely and the first start you are good to go, until you cut the car off and restart, back to the same fat condition and popping.

Bad O2 sensor? WUR? AAR doing something crazy? It has to be something electronic that re-measures after it warms up and changes setting, I guess.

Is this on your '87 930? Do the 87's have a "frequency valve?" If so, unplug it when you try to restart in hot condition.
If the car starts easier, chances are you will need to adjust the idle screw on your air meter(back it out a full turn)

[flightlead404]

When it re-starts very rich, if you just let it idle for a few minutes, does it correct itself?

[1963SWC]

Originally Posted by flightlead404

When it re-starts very rich, if you just let it idle for a few minutes, does it correct itself?

If you let it idle, it never clears out. You can try to drive it but is pops and spits and sputters so bad it is un drivable. It only runs good again if you let it sit until either the fuel pressure bleeds off, which means hours or the temp of the motor cools all of the way down.
Have you had any experience with the thermal time sensor, I think that is what it is called. Located in the left side cam chain cover?
I am going to start to diagnose these issues one by one and see if I can start to eliminate items. I have a O2 sensor on the way but don't see how this could be the issue.

[1963SWC]

Originally Posted by BauerR

Is this on your '87 930? Do the 87's have a "frequency valve?" If so, unplug it when you try to restart in hot condition.
If the car starts easier, chances are you will need to adjust the idle screw on your air meter(back it out a full turn)

Yes this is on my 87 930. Actually, before this happened, I had the car set up perfect. After warming up, set the air idle mixture screw so I had a 900 Idle for the summer here in Texas for my AC.

Do you guys think that I could have a corroded connection in the fuse box on one of the relays? If so, why does it only malfunction after you restart the car?

I drove the car today for 30 minutes, never missed a lick. Stopped, cut the ignition of and immediately restarted the car and same fat condition immediately. Tried to cycle the switch on and off about 5 times to see if that did anything.....no change.....FAT!!!

[1963SWC]

Originally Posted by 908/930

Long shot, check if your cold start injector is leaking, or just unplug it and see if there is any change.

Thank you for the idea. I will add this to the list of things to test and eliminate.

[BauerR]

Originally Posted by 1963SWC

Do you guys think that I could have a corroded connection in the fuse box on one of the relays? If so, why does it only malfunction after you restart the car?

I drove the car today for 30 minutes, never missed a lick. Stopped, cut the ignition of and immediately restarted the car and same fat condition immediately. Tried to cycle the switch on and off about 5 times to see if that did anything.....no change.....FAT!!!

I have an 84 930, so not nearly the amount of electronics as the later cars. However, I do also have a US '81 924 Turbo which, weirdly, is much closer to that of the later 930's.
With all that said, I had the same exact symptoms you do in your 930. The car was almost impossible to restart when hot.
A couple of adjustments on the idle mixture and all was perfect moving forward.
I wouldn't throw parts at it. These cars are pretty basic

[flightlead404]

Originally Posted by 1963SWC

Do you guys think that I could have a corroded connection in the fuse box on one of the relays? If so, why does it only malfunction after you restart the car?

corroded connections, especially grounds, are common. As are corroded or dirty pins on connectors and relays and the bullet fuses. Finally the split pin design on the some of the connectors and the relay bases lend themselves to loosening or failing over time.

That's why I suggested taking a few hours and checking them all, including the stuff under the drivers seat, the 14 pin connector on the cross member, and the one on the rear engine electrical panel.

[gsxrken]

Sounds to me like you’re dropping one fuel pump on the restart. Like either a red relay or the pump itself cant overcome the requirements to get the pump spinning again.
When it happens, you should try to determine if can definitely hear both pumps, and see if the car dies when you pull one relay but not the other. If you have spare relays I’d try them for an easy check, but as flightlead said you should also get after all your split pin relays, widening them with a razor blade or the like, and perhaps disassemble and clean your fuel pump wiring connections.

[1963SWC]

Originally Posted by gsxrken

Sounds to me like you’re dropping one fuel pump on the restart. Like either a red relay or the pump itself cant overcome the requirements to get the pump spinning again.
When it happens, you should try to determine if can definitely hear both pumps, and see if the car dies when you pull one relay but not the other. If you have spare relays I’d try them for an easy check, but as flightlead said you should also get after all your split pin relays, widening them with a razor blade or the like, and perhaps disassemble and clean your fuel pump wiring connections.

Thank you for that Idea. I have heard this before and this is something that I am going to check. So you think that low fuel pressure would make the mixture fat? I guess the control pressure would drop and thus make the air flow pull on the flap easier and thus make for a richer mixture.

[tops911]

check the power to the WUR, I had an issue when I swapped out the black relays that came with the car when I bought it for the "correct" red relays (I now know the new relays are junk). After checking fuel pressures with the gauges I realized that the fuel pressure stayed at the cold PSI and did not increase as the engine warmed up. I chased the fueling issue for months before swapping back to the black relays and presto the car ran at it should. The lack of power to the WUR never heated up the WUR and the car constantly ran as if it was cold.

[mark houghton]

All suggestions are worth tracking down. Here's one more possibility, based on your symptoms of the engine 'popping' and unburned fuel smell. Perhaps it's ignition related as in a heat soaked CDI giving you really crappy spark. Try sitting a baggie of ice on it directly after shutting down and give it a few minutes to cool down the CDI before restarting. Easy enough to rule that out as a possibility.

[flightlead404]

Originally Posted by tops911

check the power to the WUR, I had an issue when I swapped out the black relays that came with the car when I bought it for the "correct" red relays (I now know the new relays are junk). After checking fuel pressures with the gauges I realized that the fuel pressure stayed at the cold PSI and did not increase as the engine warmed up. I chased the fueling issue for months before swapping back to the black relays and presto the car ran at it should. The lack of power to the WUR never heated up the WUR and the car constantly ran as if it was cold.

The WUR doesn't get a red relay. Only things that have an inductive load need those. From memory, the fuel pumps. Can't think of anything else offhand although maybe I'm missing one.

[tops911]

I dont know the wiring but I know I was not getting power to the WUR when I had bad fuel pump relays (brand new red ones), put the old black ones back in and WUR got power and started to get power to it and it would warm up at it was supposed to. But it's been so long ago maybe I changed out one of the black relays at the same time? Anyway that was my experience with my car running rich, no power to the WUR and it was relay related.

[1963SWC]

Update: Not much help here though.

Hooked up gauges and found one thing interesting. Control pressure is 3.4 bar cold and goes to 3.9 bar after 4 minutes. So, it appears that I have no cold start lower control pressure today. Also today, the car would not start at all.

System pressure is 6bar. After 15 minutes of sitting, residual pressure remains 1.8 bar.

The AAR checked out good today. Took about 2 minutes to fully close so that is off the list but it brings me to another question for you guys. With the key in the on/run position, I have no power to the WUR or the AAR. If I crank on the motor, both power up, so I assume there is some type of rpm related switch to power up both of these items????

[tops911]

you can unhook the electrical connector at the back of the intake and the fuel pumps will run with the key in the on position and that could allow you to test the power to the WUR and see if it's changing the fuel control pressures, I may have done that way back when I was chasing a rich issue but I can't remember (i've hit my head a few times since then).
Good luck

[Bucketlist]

Originally Posted by 1963SWC

Update: Not much help here though.

Hooked up gauges and found one thing interesting. Control pressure is 3.4 bar cold and goes to 3.9 bar after 4 minutes. So, it appears that I have no cold start lower control pressure today. Also today, the car would not start at all.

System pressure is 6bar. After 15 minutes of sitting, residual pressure remains 1.8 bar.

The AAR checked out good today. Took about 2 minutes to fully close so that is off the list but it brings me to another question for you guys. With the key in the on/run position, I have no power to the WUR or the AAR. If I crank on the motor, both power up, so I assume there is some type of rpm related switch to power up both of these items????

Basics: Test by disconnecting the green plug from the AFM and turning on the key which will allow your fuel pumps to run. Always have the battery on a charger.
To check your cold pressures let the car sit overnight and unplug the WUR. Depending on ambient temp cold pressure should be 1.9 to 2.7 yours are too high. Then plug in the WUR. I always give mine 10 minutes for the wur to warm up and stabilize. Test the warm Control pressure should be about 3.65. Your WCP is also high.
Make sure both of your pumps run, and also test the wur at 12 v and resistance of 18-22 ohms.