Difficult starting - 1993 190E 1.8L W201 - help !

[kepla]

Okay, the car is a 1993 RHD 1.8L 190E with only 20624 miles on the clock.
About 3 weeks before Christmas it became difficult to start if it had not been used for a few days, but fine when used everyday, and would not idle properly or accelerate without mis-firing untill it had a few high-speed miles. Then everything fine untill left for a couple of days again.
After being 'rested' over the Christmas break, it will now only start if churned over for maybe ten minutes while pumpng the throttle. Once started, it just falters and dies if I let the revs drop below 1500. Attempting to drive off is pretty much impossible - put it in gear and it just falters and stops (it is an auto trans).
Last weekend I managed to keep it running long enough to get up to 90C on the temp guage, but it would start hot any easier.
I've removed the air cleaner and checked that everything looks clean and not stuck. I also (with difficulty) checked the connections to the OVP relay (and tested the fuse) and the control boxes behind the battery.
I am now pretty much at a loss as to what to check next - the electrical controls for the mechanical fuel injection seem pretty complex and I could do with some guidance on how to ensure everything is working properly.
I would much rather use logic to discover where this problem lies, and not just start fitting expensive replacement parts willy-nilly if this is possible.
Anyone got any good ideas where to look next ? Not just what to look at, but where to find things and how to check them.

[ebennz]

hello

start with you spark plugs and wires, cap and rotor, fuel filter, air filter, when were these ALL last replaced?

fuel pump may be worth checking also.

[kepla]

Thanks for the ideas.
Dunno when the ignition leads / cap / rotor arm were replaced, but I gave them a good look over and they appear to be in very good condition - no cracks or burns etc. Spark plugs were done 2 years ago, but have only covered 5000 miles or so. The are a nice coffee colour with no electrode burning, but I might still try replacements if nothing else fixes the problem.
Air filter looks consistant with about 5000 miles motoring - certainly not bad enough to stop the car from firing up, but the car won't start with it removed either - so no joy there !
Fuel filter, I don't know - I've yet t find it plugs stink of petrol after trying to start the car so I guess fuel is getting through ? Fuel pump runs for a couple of seconds when ignition is first switched on and, if the engine has actually run (a rare event) then removing the fuel filler cap gives a noticeable 'hiss'.
I also checked that the throttle switch was operating okay as I guess this somehow affects the idle circuit. Closed with throttle and open-circuit when throttle depressed. Voltage at the plug that goes on the switch, with ignition on, was 4.65 volts.
Acting on a rumour that the four-terminal coolant sensor might be faulty I checked the rsistance across diagonal terminals. 4620 ohms one way and 4590 ohms the other, at today's ambient temperature -I'd guess about 10 c.
So I'm still looking for the fault and trying to avoid throwing money at it without knowing what is actually wrong.

[ejsharp]

I would check out he OVP (over voltage Protection Relay and the fuel relay. Sometimes the circuit board on the fuel relay developes circular cracks on the back of the board where the compnent pins are soldered to the board. Solution is to drop a spot of new solder on the cracks or buy new relay.

If its the OVP then one of the lights on the dash will come on when key is activated... I forget which one... maybe the one that has something to do with the air bags.

[kepla]

Thanks, ejsharp, I spent last weekend finding the OVP relay and found it to be fine. Great conections and the fuse tested out okay. I believe that if it was an OVP relay problem then I would not have got the voltage at the plug which connects on to the throttle switch. Yes ?
When I switch the ignition on, the fuel pump runs for a couple of seconds and then stops. I believe that this is the correct behaviour and is known as 'priming'. WHen the engine actually starts (very rare) then the fuel pump kicks in again. My guess is that this show the fuel pump relay to be doing its job - yes ?
So, still no closer to having the 190E running properly again. It should be my 'daily driver' but is currently my 'daily drive ornament' !

[ejsharp]

No and no.

If the relay board has cracks then it reacts differently depending on ambient temeratures. The cracks are so fine that the solder expands and contracts according to the temperature and the temp is determined by what the engine is doing... running, not running, running hard, summer or winter etc. When expanded the current flows and when contracted... no current flow.

To test pull the relay... pull off the cover from the bottom plate and examine back of board with a magnifying glass. Look for circular cracks around the pins on the back of the board.

Regarding OVP. The fuse is no idicator of the condition of the relay. Its there to protect the relay against nasty events. The relay usually goes bad without affecting the fuse. The primary indicator of a bad OVP is the light for the air bags etc.