I have a 1993 940 Turbo with B230FT engine. It has about 232K miles. 3 months ago it stalled as I stopped at a stop sign. I made a phone call, came back and it started up easily and I drove away. Since then this has happened about once every 3 weeks. Now I've learned that if I wait 4 measured minutes, it will always start and the problem will most likely not happen again for weeks. Usually it stalls at idle, but it did happen once on the freeway.

One expert mechanic told me that this sounds like a textbook description of a fuel pump relay failure. However, my fuel pump relay does not get hot when driving. Another expert mechanic told me this is most likely an igniter problem and suggested I carry my spark tester with me to see if I have spark when it stalls to distinguish between spark and fuel causes, which I will do.

I am having another problem passing the California nitrous oxide smog test at 15 mpg and do not know if the two problems are related. The max NO allowed to pass is 486 parts per million. My first test was 705. Replacing all the spark plugs and changing from 87 octane to 89 brought the second test down to 560 ppm. Cleaning the MAF sensor brought the third test down to 532 ppm, which is where I am now.

Any insights on either problem will be appreciated.

 

Fuel pump relay.

 

Warmth is not a good test of the fuel pump relay. Next time it stalls and won't start, check to see if the fuel pump runs when you turn the key to the II position. You should be able to hear it run for around one second and then it should shut off. I make it a habit to listen for this every time I start the car. If you don't hear it then the car won't run.

My thinking is that more than likely it is a crank position sensor problem. The tend to fail intermittently - mainly when they warm up. It is located on the top of the bell housing. It has a black coax cable running from it up to a connector on the firewall. If the insulation is cracked and flaking then it probably needs replacing.

High nitrogen oxides can be caused by exhaust leaks upstream of the oxygen sensor. Check for exhaust leaks at the manifold and around the turbo. If yours is a California car, then you have a PVC valve. If it is clogged you can get elevated nitrogen oxides. Use a mighty vac to pull a vacuum on it. The car should nearly stall when a vacuum is pulled. If it doesn't then it is probably clogged and needs cleaning out/replacing.

 

I have a significant exhaust manifold gasket leak at the front cylinder. Thanks ACT1292 for your tip. The estimates for professional repair are $300-$350. I can do it for less than $30 for gaskets, but I am very concerned about warnings I've read that the manifold studs break off. If that happens, I suspect I will not have the tools to remove them. How often does that happen? More like 20% of replacements or more like 80% of replacements? Are there any other significant impediments to success for this job? Is there anything else that should be done along with this job (especially while the turbo is out)?

Secondly, I am also curious about whether my explanation of why this leads to high NOx emissions is correct. According to the Wikipedia article for catalytic converters, "Under lean engine operation, there is excess oxygen [inside the catalytic converter] and the [chemical] reduction of NOx is not favored." In my case, the excess oxygen entering the converter is due to the leak and is not caused by lean engine operation, but the effect is the same. The oxygen sensor downstream of the leak would detect an "apparent" lean engine operation and signal the ECM to enrich the air/fuel mixture, but is unable to overcome the flood of oxygen from the leak and only results in wasting fuel. Is that correct?

 

By any means I am no expert on this but I would agree with your logic. My thoughts was your poor mileage was due to the exhaust leaks. Since it is dumping excess fuel that isn't getting burned, perhaps it keeps your catalytic convertor from doing its job. Either way, the exhaust leak needs to be fixed prior to smog testing.

Check out Brickboard. There have been some pretty thorough writeups on the diagnosis of smog issues. Also Turbobricks is another source of good info for passing smog.

 

Replacing the exhaust gasket is not hard especially on non turbo cars. As far as breaking nuts/studs, that depends on how much rust is on them, etc. Best method to use for breaking the rust is heat--at least get the car hot before removing the nuts. Penetrants like PB Blaster are good but don't seem to be all that helpful in this case. Still, it doesn't hurt to soak the nuts for several days prior. If you do break a stud, don't despair, one missing won't make that much difference. I know, not perfect...

 

Surprisingly I haven't really had any trouble with breaking exhaust studs. I have replaced the exhaust gaskets on three 240s and the only problem I have had was the stud coming out of the head a couple of times. And I live in the rust belt.

 

After replacing the exhaust manifold gaskets I passed the California smog test. The maximum NOx allowed is 486 parts-per-million. I passed but was disappointed because I thought after all my work I would be done to the average NOx of 241 ppm at 15 mph. I passed at 485 ppm, one ppm below the max! So I still have a high reading and do not know the cause.

My overall high NOX emission experience has been that there was no single cause:
Replacing spark plugs, changing from 87 to 89 octane, cleaning idle air valve: down 145 ppm
Cleaning mass air flow sensor: down 28 ppm
Replace exhaust manifold gaskets: down 47 ppm

Another part of my experience: professional mechanics have real trouble diagnosing the specific cause of high NOx emissions. They can tell you ten possible causes, but don't seem to know any method for diagnosing the specific cause(s) for the vehicle in front of them. They are into replacing parts one after the other, a method I have little respect for.

My biggest concern was that the exhaust manifold studs or down-pipe studs would break off. Feedback from act1292 and lev gave me courage. None of mine broke off or were even difficult to get out. I'm in California, so very little rust on them.

My 1993 940 had a Mitsubishi TD04H-13C turbocharger. It is an understatement to say it was not easy to get this turbo out or back in when replacing the exhaust manifold gaskets. (By the way there are no gaskets between the turbo and the exhaust manifold or between the turbo and the exhaust down-pipe. )

Thanks all for your help.

 

Thanks to act1292 for this test, which solved the stall problem: "Warmth is not a good test of the fuel pump relay. Next time it stalls and won't start, check to see if the fuel pump runs when you turn the key to the II position. You should be able to hear it run for around one second and then it should shut off. I make it a habit to listen for this every time I start the car. If you don't hear it then the car won't run." Also to migbro for the correct diagnosis.

The 4-minute stalls became more frequent after 6 months of stalling about every 6 weeks. During one of those stalls, I used the test above and could not hear the fuel pump relay click or the pump come on by turning the key to the position right before "start". I replaced the fuel pump relay and that solved the problem. Grateful.