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Yundar
Joined: 13 Jun 2020 Posts: 4 Location: Glacier Washington
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Posted: Sat Jun 26, 2021 4:12 am Post subject: Control pressure too high. |
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Hi All, I am currently chasing drivability issues in my 80 NA. As I am not having much luck tuning the mixture to get the car to run properly I decided to do a pressure test. My cold control pressure seems too high. The book I am using recommends about 25 psi for cold control pressure. I am at 50 psi for this reading with the car cold and ambient air temp at 80f. I see lots of suggestions for what to do if control pressure is too low but nothing addressing if its too high. System pressure is reading a little low at 60 psi when it should be up around 65-75 psi. The car is hard to start whether warm or cold. It idles fine once it is going but has a stuttering issue and a full throttle bog. It is running very rich as evidenced by the black soot on the bumper and taillight. I have tried adjusting the mixture using the dwell meter on the frequency valve port. I can get the dwell set right at 45 but the frequency valve will cycle off dropping it to around 10. Any hints on where to go with control pressure would be great! Thanks, Andy |
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924RACR
Joined: 29 Jul 2001 Posts: 8794 Location: Royal Oak, MI, USA
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Posted: Sat Jun 26, 2021 9:00 pm Post subject: |
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Well, the control pressure is controlled by the warm-up regulator (on the back of the intake).
There is the possibility that the return line from that regulator to the fuel distributor is clogged, which would reduce its ability to bleed off the pressure - but that's unlikely. Could check it with compressed air, if you have a source.
Most likely the problem lies with your warm-up (aka control pressure) regulator. Some have had success cleaning the screen on the inlet, you might search on the board, I think there are some pics posted.
If that weren't to provide relief, you'd probably have to get it rebuilt; I've been very happy with the rebuild services of Special-T-Auto:
https://www.specialtauto.com/ _________________ Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype |
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Yundar
Joined: 13 Jun 2020 Posts: 4 Location: Glacier Washington
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Posted: Sun Jun 27, 2021 4:40 am Post subject: |
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Thanks Vaughan. When I put the car together the first time I did rebuild the WUR including removing the screen. There is a chance that something is wrong in the WUR still so I pulled it back out of the car. This seems to be the route I am on for now so I will let everyone know what the outcome is. I think the cold start issue is a separate problem and I will address that next. I can make the cold start valve operate by applying 12 volts with a little jumper box specifically made for this. It leads me to believe I'm not getting power to the CSV either due to faulty thermo time switch or no power to the thermo time switch. After all that it's on to making sure the frequency valve is properly functioning! |
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Russmw
Joined: 06 Oct 2015 Posts: 13 Location: Oxford, UK
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Posted: Tue Jun 29, 2021 11:41 pm Post subject: |
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If your cold control pressure is to high (should be about 20 psi at 20C) the car wont start as the injectors wont get enough fuel and your mixture will be all over the place. Presuming the WUR or return line isn't blocked you can tap the pin in on the WUR to lower cold control pressure.
Start here although there are a load of guides on line as the same system was used on loads of BMW\Mercedes\VW\Porsches
http://www.924board.org/viewtopic.php?t=35734 _________________ 1982 924 2.0 Lux
(Alpine White) |
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