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Old car guy.....
 
Hot Euro's Avatar
 
Join Date: Mar 2014
Location: Lubbock, Texas
Posts: 585
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Control pressure lower at cruise

As per my signature I have a modified 3.0 turbo with cis and BL wur.
Cruising home from a car show about 20mi 105* 75mph 3000rpm running the A/C (thank god). I noticed my in dash control pressure gauge reading 38-40 psi. At warm idle it's 53 psi. My A/F gauge was reading a steady 13.5 which is pretty normal for me because I like to keep it a little rich.
I have never noticed a cp reading this low at cruise (but may have never looked). Not under boost conditions but these pressures remind me of boost enrichment.
Does this sound normal to you?
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Steve
1981 SC Steel Widebody, DP Lid, 930/51 to 3.2l, K27 7006 Turbo, DOD, P&P Twin Plug heads, Twinfire Ignition, Ruf Intercooler, Powerhaus headers, Zork, CIS Euro FD, 009 injectors, 044 pump, BLwur, 930 4 sp LSD, Mocal 44 w/fan, LM2, Brembo, Retroair, Euromeisters
2003 BMW Z4 Daily Driver & Autocross car.
Old 07-22-2018, 05:37 AM
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Crotchety Old Bastard
 
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WCP should be around 53psi as you listed. If the AFR at cruise has not changed then the WCP has not changed. A reduction from 53psi to 38psi is very significant. I'd get a second opinion on the fuel pressure reading.


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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 07-22-2018, 10:31 AM
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Quote:
Originally Posted by RarlyL8 View Post
WCP should be around 53psi as you listed. If the AFR at cruise has not changed then the WCP has not changed. A reduction from 53psi to 38psi is very significant. I'd get a second opinion on the fuel pressure reading.
.
Thanks Brian, this does verify my thinking. When I noticed the low control pressure I expected to see a drop on my af gauge.* 13.5 to 14 is what I am used to at most any speed. When I got back into town in stop and go driving the fuel pressure had recovered. I expect this may be a gauge or sender problem. May be time to upgrade to a better quality set.
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Steve
1981 SC Steel Widebody, DP Lid, 930/51 to 3.2l, K27 7006 Turbo, DOD, P&P Twin Plug heads, Twinfire Ignition, Ruf Intercooler, Powerhaus headers, Zork, CIS Euro FD, 009 injectors, 044 pump, BLwur, 930 4 sp LSD, Mocal 44 w/fan, LM2, Brembo, Retroair, Euromeisters
2003 BMW Z4 Daily Driver & Autocross car.
Old 07-22-2018, 11:39 AM
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Crotchety Old Bastard
 
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Here is a video of warm-up AFR vs Control Pressure. It's like watching paint dry so click on the time line at one minute intervals for an overview of typical readings. This was done in the winter, summer warm-up is of course much quicker.

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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 07-22-2018, 04:14 PM
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I always enjoy seeing that video of your warm-up, this inspired me to add my fuel pressure gauge. this picture is just to show my gauges taken just after a warm weather start.

I wish I had the camera ready to film the malfunction - may have seen something on the voltmeter.
At the time of my low fuel pressure reading I was running the a/c blower on high, the condenser fan and the oil cooler fan. Maybe I was just passing my max alternator output. I also saw my temp gauge starting to wander. Not sure why everything showed normal after slowing down in town.
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Steve
1981 SC Steel Widebody, DP Lid, 930/51 to 3.2l, K27 7006 Turbo, DOD, P&P Twin Plug heads, Twinfire Ignition, Ruf Intercooler, Powerhaus headers, Zork, CIS Euro FD, 009 injectors, 044 pump, BLwur, 930 4 sp LSD, Mocal 44 w/fan, LM2, Brembo, Retroair, Euromeisters
2003 BMW Z4 Daily Driver & Autocross car.
Old 07-23-2018, 03:37 AM
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I had a electric fuel pressure gauge in the dash and found the reading to be voltage sensitive. As the battery was charging while driving the fuel pressure reading would increase. I'm wondering if some type of voltage stabilizer for the gauges would give a more accurate reading.
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'79 930/DP935
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Old 07-23-2018, 11:51 AM
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Quote:
Originally Posted by gumba View Post
I had a electric fuel pressure gauge in the dash and found the reading to be voltage sensitive. As the battery was charging while driving the fuel pressure reading would increase. I'm wondering if some type of voltage stabilizer for the gauges would give a more accurate reading.
I remember back in the day, maybe before computers, some cars came from the factory with a voltage regulator in the dash for the gauges. I might have one and not even know it. Anybody up on this?
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Steve
1981 SC Steel Widebody, DP Lid, 930/51 to 3.2l, K27 7006 Turbo, DOD, P&P Twin Plug heads, Twinfire Ignition, Ruf Intercooler, Powerhaus headers, Zork, CIS Euro FD, 009 injectors, 044 pump, BLwur, 930 4 sp LSD, Mocal 44 w/fan, LM2, Brembo, Retroair, Euromeisters
2003 BMW Z4 Daily Driver & Autocross car.
Old 07-23-2018, 03:32 PM
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