hubturn

95 Cab with 12v. Car is overheating. Replaced the thermostat and removed the water pump. The pump seemed OK, not leaking and the impeller turned when I turned the drive pulley. Put it all back together, flushed the heater core and radiator, both of which I replaced within the last three years. Just to check, I bypassed the heater core with clear tubing, and the radiator with clear tubing. Clamped off the expansion tank overflow hose. I wanted to SEE the fluid flow. Brought the temp up to the first big line. Here's what I have. No flow through the bypassed radiator hose, and bubbling up boiling water in the bypassed heater core hose. I attempted to bleed out the air, but there is no sustained forceful flow of fluid that I would expect with a turning and pumping water pump. I have difficulty believing that the water pump is bad. I'd appreciate your thoughts.

Rusty Spokes

Best guess of mine would be air in the block. When I did the water pump and radiator on my 98 Cabriolet it took a while to get all the air out of the cooling system.

There should of been a jiggle valve in the thermostat..that helps work the air out of the block when you refill the system. Did your thermostat have one? This picture is from a Toyota..Audi uses a simular jiggle valve.



I also have a cooling system pressure tester. When I refill the system with antifreeze I will leave the bleeder valve open and then use the pressure tester to pump antifreeze into the system, prior to starting the engine..till coolant flows out of the bleeder. Even after doing this I get a tremendous drop in the coolant level in the overflow tank when the thermostat opens (air bubble in the block)..and it still takes a while for the hot antifreeze to flow bubble free from the bleeder valve on the heater hose by the firewall.

See if you can find a cooling system pressure tester with the correct adapter for VW/Audi. You need something like this:

hubturn

Thanks Rusty. Interesting approach of forcing fluid in the system via the pressure tool and essentially bleeding the system while cold as much as possible. I like that. What I was hoping for was confirmation that with the heater core bypassed with a clear hose, I would see continuous flow via pressure from the water pump, indicating continuous circulation of the coolant. I belive that should occur with or without the thermostat open. I suppose that if there is any air in the system, the pump could be cavitating, but I can't be sure about that. Alternatively, I was hoping someone would say that you can have a sheared water pump shaft and that it would appear to still be functional when removed from the car, (i.e. a misdiagnosis on my part and a bad pump in fact). Lurking in the back of my thoughts is fear of plugged coolant passages in the block.

Rusty Spokes

My thinking is your water pump is captivating due to air. You had the water pump off and it passed inspection so it would seem to be an air problem..a water pump wont push air.

That is why I asked about the jiggle valve in the thermostat..it allows antifreeze to work around/bleed into the water pump and get things moving.

1987-audi-owner

Hey!! I also have an overheating problem. I own an 1987 audi 5000 cs turbo. As soon as the engine gets hot and I turn the car off it will not start again... As soon as the engine has been given time to cool down it starts no problem.. Any suggestions?? Thanks!

Rusty Spokes

Is the engine actually overheating? Running very warm on the temperature gauge? Or, is the engine just not starting once at normal temperatures?

1987-audi-owner

It is staying at the normal temperature. (technically not overheating) but it wont start once the engine gets warm/hot

Rusty Spokes

Could be a couple of things..You have C.I.S. injection and C.I.S. hates vacuum leaks. You may have a lean fuel condition when the engine is at operating temperature.


First...Check all the hoses, boot(s) and injector seals..be sure there are no air leaks.

If you are confident the system is without air leaks..Second...check the sensor plate..the big round thing under the air boot next to the fuel distributor. With the engine off and at operating temp..see if the sensor plate will move up without dragging on the sides of the taper..it should be free with some resistance pushing up from the air filter side. You should hear the fuel injectors squeal when you push the sensor plate up, engine warm and engine off. Don't get too carried away..you can flood the engine.

Third...The fuel system should hold pressure for several hours after turning off the engine. There is a check valve in the fuel distributor and fuel pump.If the fuel system is not holding pressure once the engine is off..you could have a "vapor lock"..the fuel boiling point is very low without the pressure...it boils and all you have is fuel vapor when trying to restart the engine.

You need some gauges hooked up to check the system...or you could..with great care about the hazards of fuel spraying around a warm engine...just check the fuel at the inlet of the fuel distributor by cracking open the big fuel line nut..engine warm and off..see if you have any pressure in the fuel system after about twenty minutes...you should have an abundance of fuel under pressure. If not you need the check valve in the pump and "O" ring in the fuel distributor.

Having the fuel mixture set with a CO machine is ideal..if there are any shops around you that have some experience with older C.I.S...have them check the CO/mixture..very important.

With C.I.S. I strongly suggest using Marvels Mystery Oil (available at most auto parts stores) in you fuel..it will help lubricate all the old "O" rings, injectors, fuel piston and seals. You need MMO now that there is ethanol in our gasoline. Ethanol is very "dry"..maybe corrosive and not very friendly to the old fuel systems. MMO will help.

Jeff-AudiUSAParts

Hubturn, the pressure bleeder is what I'd suggest also. I have one and used it on friends Porsche 944 and you have to do it to get air out of the system.

So on your car there is a bleed valve/plug on the pipe on the back of the engine. If my diagram is right it's on or toward the drives side on top of the pipe. I'd pressurize the cooling system with the hand pump to about 14 psi and then slowly turn the plug out of the pipe. Hopefully the plug has a hole in the side so air pockets bleed out until you see coolant. What you don't want to happen is that plug pop out under pressure then splatter coolant all over the place. Do all of this with the engine cold.

Jeff-AudiUSAParts

87 Audi owner, did you replace the spark plugs recently? I ask because I helped out a friend that put in cheap platinum plugs and he created his own hot start problem by not putting in a better line of spark plugs. For some reason his Audi didn't like platinum plugs and reacted the way yours is.

I've had many rabbits with CIS, and rusty spokes is dead on with other CIS fuel issues. Vacuum leaks at the rubber injector O rings going into the head are another area you should look at. Your car has at least one fuel pump check valve, one of which should be on the fuel pump. If that goes bad the system doesn't hold residual fuel pressure for a certain period of time and creates that hot start problem. The warm up regulator bolted to the side of the block is what controls system and control fuel pressure and if those aren't in spec you can have problems like this too.