Warm Start Problem
08-10-2013, 09:38 PM
#1
Track Day
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Warm Start Problem
1978 5 speed
if it's cold - no issue
at first i thought it might be some sort of vapor lock (and maybe it still is) - had the issue several times when i had probably less than 1/8 tank.
however today - same deal and had filled up few hours earlier.
it's not an everytime issue - but - sometimes when warm - it will crank but never start. car then has to sit for several hours - and then - bang - no problem.
i'm stumped on this one.
if it's cold - no issue
at first i thought it might be some sort of vapor lock (and maybe it still is) - had the issue several times when i had probably less than 1/8 tank.
however today - same deal and had filled up few hours earlier.
it's not an everytime issue - but - sometimes when warm - it will crank but never start. car then has to sit for several hours - and then - bang - no problem.
i'm stumped on this one.
08-10-2013, 09:53 PM
#2
Burning Brakes
Hi,
I have a similar issue, when cold perfect start, when hot, meaning 5/10 Minutes after shut down also perfect start but when warm, about 30 minutes or a few hours more and it only starts with fully depressed acelerator pedal.
According to the Rennlist specialists i have leaking injector(s).
However if it doesn't start in any maner when warm it could be something else, better wait for a really expert opinion, I'm just here learning everyday!
Regards
I have a similar issue, when cold perfect start, when hot, meaning 5/10 Minutes after shut down also perfect start but when warm, about 30 minutes or a few hours more and it only starts with fully depressed acelerator pedal.
According to the Rennlist specialists i have leaking injector(s).
However if it doesn't start in any maner when warm it could be something else, better wait for a really expert opinion, I'm just here learning everyday!
Regards
08-11-2013, 02:32 AM
#3
Rennlist
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First step:
Fuel pressure guages hooked up to measure system and control pressures....and to help isolate where the fuel is leaking.
Almost all hot start problems are because of the loss of pressure. The trick is to figure out where the "fuel leak" actually is.
External fuel leak
Fuel pump check valve
Accumulator leak
Fuel distributor leak
Warm-up regulator leak
Cold start vavle leak
Fuel injector leak
Fuel pressure guages hooked up to measure system and control pressures....and to help isolate where the fuel is leaking.
Almost all hot start problems are because of the loss of pressure. The trick is to figure out where the "fuel leak" actually is.
External fuel leak
Fuel pump check valve
Accumulator leak
Fuel distributor leak
Warm-up regulator leak
Cold start vavle leak
Fuel injector leak
__________________
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
08-11-2013, 10:00 AM
#4
Rennlist Member
Had similar in 83 CIS. Changed accumulator, seemed to be fixed, but returned. On some CIS there is a temp sensor whose contacts are closed when temp is less than full hot, but not low enough for the WUR to play much of a role (forget the details) - its fitted to the hot water outlet on the rear of the RH head. This is powered ONLY while cranking - if closed, it powers a solenoid nearby that lowers control a bit to richen the mixture a touch, to assist starting. There is a test listed in the WSM. I did this, and the solenoid didnt open. I pulled the sensor, and when the temp was in range, it read 20 Ohms - high enough to prevent the solenoid firing. Some US outlets stocked the sensor, but wanted 3 times its cost to ship it to Oz. I found a 924 version on a US site at a sensible price + shipping total. This unit had a M10x1 thread, instead of the original M14x1.5, so I had to make up an adaptor - no problem as the original hole was blind, and I already had the necessary taps. Solved my problem.
Yours may be a different issue, but if you have this late CIS fitting, its worth checking.
jp 83 Euro S AT 55k
Yours may be a different issue, but if you have this late CIS fitting, its worth checking.
jp 83 Euro S AT 55k
08-11-2013, 11:31 AM
#5
Burning Brakes
As mentioned, you need to hook up a fuel pressure gauge.
My '82 had the hot start problem. As soon as the fuel pump turned off, my pressure instantly dropped to zero. I isolated the problem by crimping off the fuel hoses leading to the injector rails and energizing the pump for a second. Pressure still dropped to zero which lead me back to the check valve on the fuel pump which was stuck open. I freed it up with a pin and cleaned it. Pressure now holds as per specs and I get instant hot starts now. Got a new one sitting here if this one decides to stick open again. Pretty cheap at $20 bucks.
Still not sure why it only affected hot starts. There's a cold start injector but you'd think it needs an instant supply of fuel ready to go too.
I've read that the accumulator is another common problem on the older models.
My '82 had the hot start problem. As soon as the fuel pump turned off, my pressure instantly dropped to zero. I isolated the problem by crimping off the fuel hoses leading to the injector rails and energizing the pump for a second. Pressure still dropped to zero which lead me back to the check valve on the fuel pump which was stuck open. I freed it up with a pin and cleaned it. Pressure now holds as per specs and I get instant hot starts now. Got a new one sitting here if this one decides to stick open again. Pretty cheap at $20 bucks.
Still not sure why it only affected hot starts. There's a cold start injector but you'd think it needs an instant supply of fuel ready to go too.
I've read that the accumulator is another common problem on the older models.
08-12-2013, 04:56 PM
#6
Rennlist Member
look in the passenger side rear wheel well and remove the plastic cover. Your accumulator resides in there. Before hooking up gauges etc...just look in there and see if you can get a visual/smell confirmation of fuel leakage. If either....that's your problem. If not...go back to gauges and start from the top...
08-12-2013, 05:36 PM
#7
Rennlist Member
Some will frown on my unscientific approach, but simply replace the fuel check valve at the fuel pump (in the right rear wheel well for mine).
It is probably leaking. It is 35 years old and it is cheap.
And it is most likely the problem.
If that doesn't work, please see Greg's response.
It is probably leaking. It is 35 years old and it is cheap.
And it is most likely the problem.
If that doesn't work, please see Greg's response.
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08-12-2013, 05:55 PM
#8
Rennlist
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Some will frown on my unscientific approach, but simply replace the fuel check valve at the fuel pump (in the right rear wheel well for mine).
It is probably leaking. It is 35 years old and it is cheap.
And it is most likely the problem.
If that doesn't work, please see Greg's response.
It is probably leaking. It is 35 years old and it is cheap.
And it is most likely the problem.
If that doesn't work, please see Greg's response.
As I've mentioned before, the 911 vehicle "ate" these check valves....we would sometimes install several, in the same day.
For some strange reason, never as common on the 928 vehicle.....but as mentioned above: "It is 35 years old and it is cheap."
08-12-2013, 10:53 PM
#9
The 78 models came from the factory with fuel pumps that had internal check valves and are not replaceable. Many have been replaced with later pumps that have external check valves. The pumps with internal check valves have a long, hex shaped outlet connector. If yours is the old internal style, try running a good fuel system cleaner though it before replacing the pump.
Also when checking the accumulator, be aware there are two types. The original had a screw in the back which can be removed to see if fuel was present. If fuel is present, the diaphram is bad. Later versions had a hose that returned any leaking fuel to the tank as a safety factor. If fuel is present when the hose is removed, the accumulator needs replaced.
Dennis
Also when checking the accumulator, be aware there are two types. The original had a screw in the back which can be removed to see if fuel was present. If fuel is present, the diaphram is bad. Later versions had a hose that returned any leaking fuel to the tank as a safety factor. If fuel is present when the hose is removed, the accumulator needs replaced.
Dennis
08-13-2013, 02:27 AM
#10
Rennlist
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The 78 models came from the factory with fuel pumps that had internal check valves and are not replaceable. Many have been replaced with later pumps that have external check valves. The pumps with internal check valves have a long, hex shaped outlet connector. If yours is the old internal style, try running a good fuel system cleaner though it before replacing the pump.
Also when checking the accumulator, be aware there are two types. The original had a screw in the back which can be removed to see if fuel was present. If fuel is present, the diaphram is bad. Later versions had a hose that returned any leaking fuel to the tank as a safety factor. If fuel is present when the hose is removed, the accumulator needs replaced.
Dennis
Also when checking the accumulator, be aware there are two types. The original had a screw in the back which can be removed to see if fuel was present. If fuel is present, the diaphram is bad. Later versions had a hose that returned any leaking fuel to the tank as a safety factor. If fuel is present when the hose is removed, the accumulator needs replaced.
Dennis
Bosch cleverly made several different check valves that can be threaded into those pumps. Simple to pry out the old internal valve, so none of the "pieces" get into the new check valves.
09-13-2013, 02:02 AM
#11
Track Day
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so - finally took car into my guys - and gave him copy of this thread to give them something to go off of...
this is response i got:
here is the breakdown on cost:
some of this stuff seems crazy expensive compared to some other things i've had done - am i getting screwed on this? or does this seem reasonable? is there a better source for parts needed than what they've got?
looking for some feedback - i thought this was going to be about a $500-$700 or so fix - not 2k
edit: the formatting is tough to get right on their quote - but the $279.44 for both above is the labor
this is response i got:
We were able to get the car in the shop this afternoon to get a look into the fuel system. We did notice prior to hooking up the fuel pressure gauges that the car has a pretty heavy misfire under load when the throttle is pressed. The spark plugs and the coil are new parts that we changed last year and they are still good. The issue is within the plug wires and the distributor cap and rotor. We pulled the wires and the 2 cylinders that are misfiring have badly worn out wires where they meet the distributor cap. For the misfire issue the car will need new plug wires, distributor cap and rotor.
The fuel pump pressure delivery is great and well within spec. The fuel system bleed down when the car is off is also still in spec but the WUR is why we have a ****ty idle when cold and believed to be the hot re-start concern as well. The WUR or warm up regulator on your car is not operating properly and that is why I wanted to quote it out for you when we did the a/c.
Having an adjustable warm up regulator will allow the car to start cold and idle normally without mashing the gas to get it to turn over, this will also help with keeping the idle when the a/c system is used and allow it to hold enough pressure to fire when it is warmed up… The WUR is not available from Porsche any longer so we have sourced an adjustable one from a 928 specialist.
The replacement of the plug wires, cap and rotor for this car is $780.95
The replacement of the WUR with and adjustable unit is $896.44 and is a week away.
Including diagnosis the total with taxes will come to just under 2 grand, I may have some misc expenses during teardown and reassembly if there are some vacuum lines that break or something so we might end up slightly over $2,000.00
The fuel pump pressure delivery is great and well within spec. The fuel system bleed down when the car is off is also still in spec but the WUR is why we have a ****ty idle when cold and believed to be the hot re-start concern as well. The WUR or warm up regulator on your car is not operating properly and that is why I wanted to quote it out for you when we did the a/c.
Having an adjustable warm up regulator will allow the car to start cold and idle normally without mashing the gas to get it to turn over, this will also help with keeping the idle when the a/c system is used and allow it to hold enough pressure to fire when it is warmed up… The WUR is not available from Porsche any longer so we have sourced an adjustable one from a 928 specialist.
The replacement of the plug wires, cap and rotor for this car is $780.95
The replacement of the WUR with and adjustable unit is $896.44 and is a week away.
Including diagnosis the total with taxes will come to just under 2 grand, I may have some misc expenses during teardown and reassembly if there are some vacuum lines that break or something so we might end up slightly over $2,000.00
QUOTE Tech Emery: found vehicle misfiring. found ignition wires faulty. Suggest 279.44 501.51
0.00 replace ignition wires, cap and rotor.
1.00 928-602-902-00 DISTRIBUTOR CAP (1 DAY) 119.00
1.00 928-602-904-03 DISTRIBUTOR ROTOR (1 DAY) 26.07
1.00 09 096 IGNITION WIRE SET MAG CORE (1 DAY) 356.44
0.00 SOLD Top off under hood fluids and set tire pressure including spare if equipped. 0.00
896.44 QUOTE Recommend installing an adjustable warm up regulator for fuel system. 279.44 617.00
1.00 928 ADJ. WARM UP PRESS REG (1WEEK) 599.00
1.00 SHIPPING 18.00
0.00 replace ignition wires, cap and rotor.
1.00 928-602-902-00 DISTRIBUTOR CAP (1 DAY) 119.00
1.00 928-602-904-03 DISTRIBUTOR ROTOR (1 DAY) 26.07
1.00 09 096 IGNITION WIRE SET MAG CORE (1 DAY) 356.44
0.00 SOLD Top off under hood fluids and set tire pressure including spare if equipped. 0.00
896.44 QUOTE Recommend installing an adjustable warm up regulator for fuel system. 279.44 617.00
1.00 928 ADJ. WARM UP PRESS REG (1WEEK) 599.00
1.00 SHIPPING 18.00
looking for some feedback - i thought this was going to be about a $500-$700 or so fix - not 2k
edit: the formatting is tough to get right on their quote - but the $279.44 for both above is the labor
09-13-2013, 03:09 AM
#12
Rennlist Member
In their defense, however, since a professional repair shop needs to minimize litigation costs, they are forced to use parts that are designated specifically for this model car. As an example, they can't use less expensive Ford Mustang injectors in a 928, even if the injectors would fit and function perfectly. The repair shop is forced to use 928 products which are normally priced at a premium.
Doesn't mean that you have to buy products that weren't originally made for the 928, though.
09-13-2013, 07:02 AM
#13
Rennlist Member
Do a search and replace the parts you can easily do yourself, that are cheaper, BUT are really proper for the job - ie, from the big 4 - leads, cap, rotor etc. If you have leads where the plug cap unscrews, you can replace only the leads you really need to keep the cost down, but it might be a false economy in the long run. I am a bit wary of 'adjustable WUR' - you can make the post that carries the bimetallic strip adjustable readily, but thats only half the problem - the cold control starting point. Making main hot control adjustable is only possible by buggering the diaphragm underneath (if fitted) to get an allen key in there, AFAIK. I suspect a 78 wont have this type of WUR, but if it does, do you want one thats been monstered? Or is there a way to make these adjustable that I dont know about , apart from tapping the cast iron part in and out?
jp 83 Euro S AT 55k
jp 83 Euro S AT 55k
09-13-2013, 09:50 AM
#14
Rennlist Member
It doesn't seem outrageous...for a shop. A good set of plug wires will run you $200~+. I bought my distributor a few years back for $60, they are charging double. That's about right for any shop worth their salt. Remember they are making money on parts, as they should. They have diagnostic time, overhead,etc involved. Now replacing all that stuff, is very easy, but that's up to you. I personally would like to see the full set of diagnostics on the system with CIS gauges installed and their respective results. The WUR can be rebuilt yourself depending on what's wrong. You have nothing to lose since a brand new one is $600. There are people out there that rebuild them too. Best of luck!