The patient: my 79 928, all stock.

Managed today to get the air leaks sorted out, meaning it now starts with barely a twist of the key, very nice. Idles at about 1400 RPM for a bit then settles down to 850 and will stay there for hours. During warm-up it will rev to the redline without any issues. Once warm it runs like crud except at idle. Surging, or starving for fuel, seems to be the problem but it could be ignition.

Where to start? Assuming the air leaks are plugged what would be the next logical step or is what I described symptomatic of a common failure?

Most of the searching I did returned the opposite issue; can't idle, can run. My can idle, can run, then warms up and runs like crap, isn't covered so much for the early CIS cars.

Thanks in advance.

 

Is fuel delivery in spec?

WUR holding vacuum?

 

Sounds like your hot control pressure is too high - BTDT! You need a gauge set to check control and system pressures - contact Roger at 928srus, he does the correct set. Your system pressure should be 65-75psi, and hot control 40-46psi. If hot control gets past ~55psi, power will go right down. Cold control can go as low as 15psi, depending on ambient temperature. Most usual cause is a blockage in the WUR inlet - multiple layers of brass gauze full of crud, the pressure regulator, the outlet fitting, or the retun line to the tank. Get it hot with the problem, (extinguisher handy!), carefully, with rag around the joint, crack open the WUR outlet union - if little fuel comes out, blockage is in WUR - engine note/rpm should not change; if much fuel, and engine note/rpm changes, blockage is further along the line towards distributor and return line. Once you have localized the problem, advise. Note that if the cold start is OK, somethings (idle, mixture) may have been adjusted to try to compensate for the high hot control, so when you fix the pressure, other adjustments may be necessary. Also useful to check fuel delivery rate is OK - open return line over the RHS cam cover, run a hose from the engine side fitting to a container, bridge the fuel pump relay, ~1350ml in 30 secs.
jp 83 Euro S AT 54k

 

JP has it right you need to measure the pressure since the control pressure regulator (WUR) is what adjusts the mixture as the engine gets warm.

 

ok i'll check the fuel return today. have to order a pressure testing kit as well to test the war, figured it was inevitable. also found the state of the plug wires is...original. i need to replace those soon.

 

Also is the WUR user serviceable - this junk inside of it I've heard described, can it be cleaned out?

 

Usually it is just in the inlet screen...if it's still there. Just went through and rebuilt mine...the Ferrari 400 guy has a rebuild manual online, but does not show vacuum controlled WUR, pretty much the same though. Just be careful taking it apart so you don't crack the bellows. Also there is a thin metal plate that is used to modulate the pressure just like in the FD...so be VERY CAREFUL...don't ask how I know...not good. Once it's all apart though just take the upper half and can usually clean the ports out with some brake clean. I let mine sit in a small can of parts cleaner for a night, next day I sprayed some brake clean in the ports and it came out the ports on the other side...so I guess I can assume they were clean, but they do not go straight through.

Hope that can be of some help.

Josh

 

If you dont want to open it up, try blowing air, or preferably a solvent IN to the outlet port, until it comes out the INLET port. Take care to catch anything that gets blown out - filters etc. If they come out, and you want to replace them, best way to clean them is apply a match (away from fuel!) to burn off the lint and crud. The insides are not rocket science, but you need to get it back together the way it was before. Do a search, there is a document of mine on landsharkoz web site.
jp 83 Euro S AT 54k

 

I believe I have arrived at a sequence of events that caused the WUR to fail. I am still going to pressure test it but it is really just for kicks, I have little confidence in the unit after visual inspection.

The car had some odd modifications to the air bypass system, presumably to make it idle better or overcome all of the other PO-ignored air leaks. Someone had created a secondary air bypass using stock Porsche parts (I had a previous thread asking anyone to I.D. this stuff since it looked stock) - Porsche parts apparently from another model. So the car had a static bypass from before the AAR to the plenum. It was good for about 500 RPM, a rather large bypass line. All I can guess is what happened after that - it was running lean so they screwed with the WUR. You can see a punch mark in the top on the plug and scrapes, presumably from whatever they used to punch it in further.

Good for a core perhaps, but I can't drive a car with a fuel component being obviously hamfisted. And so it goes. At least the fuel distributor appears untampered. The WUR is reasonable enough to not waste hours on.

Ordered a pressure test kit today to get the rest of it sorted out with a new WUR.

 

You can re-adjust the barrel pin in the WUR by disassembling the unit and driving the pin back up. Or drive it completely out the bottom, clear and clean the bore, then drive it back in from the topside. Good starting point is .085-.090" sunken from the top surface.

 

Search also on this site for a link to a company that does economic refurbs of WUR. I have one that has been modified so you can pull the post up with a bolt.
Its not worth even starting on adjusting mixture or WUR setting until you get system pressure right, then control pressures, then main mixture.
ONCE it is right though, keep up regular use, and with good filters it should last a long time - My last WUR issue was 10 years ago now, and it never misses a beat, pulls like a train hot or cold.
jp 83 Euro S AT 54k

 

Roger is sending me a test kit today, and a replacement banjo bolt for the WUR as mine was twisted from over-tightening. Next week I'll have time to replace the fuel lines and then test everything from the *** end of the car to the front.

It makes me crazy to have this monster in the driveway and I can't go down the street.

 

I'd start with the basics before going into any 928 OB/CIS specific components. Does it have good spark? Is airflow restricted in any way? Is the throttle plate adjusted properly? What vacuum does it make at idle and is it within spec? Is fuel pressure up to spec? Does it hold residual pressure?

Guys, is the WUR just different nomenclature for a typical IAC?? If so then check the IAC/WUR air passages for carbon buildup and that the stepper motor isn't crudded up with carbon. Is the vacuum circuit providing the pressure regulator with vacuum leaking? That controls fuel pressure to compensate for injection during idle (requires less pressure due to vacuum) and WOT (requires more pressure due to no vacuum).

How is the fuel in the tank? Has it been sitting for a while? Do you commonly keep it at no more than half a tank (could be water settled into the bottom from condensation)? When tanking up do you top it off to get every last drop of fuel into the tank (this can get rid of the approx. 12% air pocket required in the system for EVAP to work or you'll flood the charcoal cannister with fuel and that could be causing it to run rich when the charcoal cannister purge solenoid opens the valve)?

 

All done. We're down to fuel delivery with this car, not electrics or air intake. It sucks and sparks, just doesn't like to go under load. Starvation is the thought but need to check fuel pressures, hence the test kit.