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The smell from underneath is quite likely from a worn and possible overworked main fuel pump. They can get very hot, baking paint and undercoating. An overworked/failing main fuel pump will draw a lot of extra current and easily take out the fuel pump relay, not to mention overheating the fuel pump fuse. By the time they get to that stage the fuel pump will likely need replacement, but you'll want to consider a number of aspects of fuel delivery before declaring the fuel pump dead as they're rather pricy to replace and you also don't want to prematurely wear out a replacement pump if other problems persist. Depending on how long you want to keep the car, a used low mileage pump can be okay, but obviously has a more limited life span.
Things to consider:
o Has the main fuel filter been replaced in living memory? Although a good Bosch filter can last many years (almost the life of the car if you're lucky) they are supposed to be replaced regularly. You can do a crude test by comparing fuel flow before and after the filter.
o There is a possibility of a blockage in the fuel pressure regulator. Additionally, a faulty regulator that sets too high a pressure can overwork the fuel pumps. A proper diagnosis requires a fuel line pressure test. The 700/900 FAQ has additional information on checking fuel pressure regulators.
o Was the in-tank pre-pump filter sock replaced? Was it clean and undamaged? Was there evidence of dirt in the fuel tank? Always inspect the bottom of the tank splash pan when removing the tank pick-up assembly. Things would have to be pretty bad, like flakes of tank rust, to restrict fuel flow into the pre-pump.
o You put in a used pre-pump. You have no guarantee it was in good shape. A worn pre-pump may seemingly run happily for a few minutes, but seize as it warms up. Running it dry on the test bench for anything more than a few seconds is a bad idea. A dead or dying pre-pump will overload the main pump which in turn overheats the f/p relay. Check for the sound of pre-pump operation when the main pump starts to run hot.
o Was the ribbed hose between the pre-pump and the pick-up tube replaced? If not, did you carefully inspect it? Even a small split would allow the pre-pump to free wheel and wear out prematurely. Again, the main pump would have to assume an extra load. For anything less than a half-tank of gas, the engine could stumble, especially as you go through a corner or up a hill. Ultimately it will stall for lack of fuel. If the main pump is worn, the same thing could even happen with a full tank. That hose can be replaced with standard fuel line. The early versions of this hose were thin and prone to disintegrating, especially in ethanol gas blends, but by '89 they were heavier rubber and do seem to last.
o This is a long shot and I don't mean to insult your skill level, but is there any chance the in-tank fuel pump electrical terminals were accidentally swapped? Polarity is marked at the base of the terminal posts. The + post gets the black wire, the - post gets the short (brown) wire to the nearby screw terminal. I would expect the engine to either not start, immediately stall or at least run like crap if this happened.
o You probably keep putting in used fuel pump relays. This tip won't solve your immediate problem, but as a preventative measure you should re-flow the solder on the main circuit board connections. Add a little extra solder to help act as a heat sink and clean the board of flux residue. Place the retaining clip on the relay case such that the circuit board underneath is toward the open side of the clip. Position the relay in a cooler, more ventilated position, not near a heat source and not stuffed in some corner amidst a bundle of wires (like in your '89).
o Is there any chance the fuel lines (including the return line) have been kinked or damaged? Any restriction will overwork the main fuel pump and/or starve the engine.
o Are the fuel pump fuses nice and clean? A poor contact, corroded or worn fuse end at the fuse panel can build resistance as it heats up. This could make the in-tank pump run slower or even stop. Caution, the main fuel pump fuse can get hot enough to burn skin.
If all other systems are okay then you may want to diagnose the fuel pump before replacing it.
o A proper diagnosis involves pressure testing at the fuel rail and requires special equipment. I think you'll find this described in the 700/900 FAQ (your LH 2.4 system is basically the same as in the 700). As a crude test you can check fuel delivery volume out of the delivery hose connected to the fuel rail. You will first want to test with the engine cool as that's the safest, but you may need to repeat the test with the fuel pump hot to re-create the problem.
o Another test that can be done is to check the current draw at the fuse panel. Your basic multi-meter usually won't have the capacity to do this test. A good main fuel pump should draw just under 7 amps, an unhealthy one will draw 9 amps or more. For LH systems, the main fuse also includes the pre-pump load. Fortunately the pre-pump has its own fuse so you can measure the current there (pre-pump should draw about 3-4 amps in your case -if you see nothing, guess what's dead again) and subtract it from the fuel pump fuse reading to get the draw for the main fuel pump. If your main fuel pump is running extremely hot then it's almost guaranteed you'll see a high draw.
Obviously, for fuel delivery tests you must be very careful working with open gasoline, especially around a hot engine or any potential spark source. You disable the engine from starting, relieve fuel system pressure, open a fuel line at a test point then crank the engine for a short period of time using a container to catch the pumped fuel. A good overall test point is hooking a length of hose on the return line out of the fuel pressure regulator. You should see a good steady flow of gas into the catch container. You can compare input/output flows around devices (other than fuel flow restrictors like the pressure regulator). I can't remember and can't find a reference to an adequate fuel delivery rate, but it's something like 1/4 cup every 10 seconds at the fuel rail intake. Maybe someone has a better number. At any rate, it is not a good substitute for fuel pressure testing.
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day
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