1989 420 SEL cranks but no start

[Cigar Havana]

This is a 1989 420 SEL, 275K KM, the engine cranks but does not start after about 6 attempts. Car has been sitting in an unheated garage for about 1 week, and today it is -7c. On first attempt, there was a loud buzzing sound coming from the front passenger foot well. So I replaced the idle control module, which is located there. No more buzzing on start attempts. Then replaced fuel pump relay (used) , OVP(new). Still cranks but no start.

Car had been idling at little too high for about 3 months, and 3K KM prior to this no start situation. At first when the high idle appeared, it was only on a warm engine, but most recently the high idle would be at both cold and hot engine in park or neutral. It was around 800 to 1000 RPM in park. Sometimes it would settle down to the desired 650 in park but at the next stop it would back up to 900 or 1000 rpm in neutral. Most recently, on cold start it would be at about 800 RPM in park when outside temperature was about 3C.

Here is repair history, most recent repairs first. Only listing ignition and fuel, vacuum repairs vacuum here.

Trying to solve high idle which was present prior to the current no start condition, replaced Idle control valve (ICV), distributor cap and rotor, which had no effect a week ago. It started well, after these parts were replaced and only driven about 30 KM afterwards.

The climate control unit was replaced with brand new one, about 1,000 KM ago. Battery replaced at 270,000 KM.

Valve job was done at 250,000 KM, 2 years ago, included new chain guides, chain tensioner, cold start valve, intake manifold 'o' rings, injector seals, new plugs, belts, new head bolts and washers, etc.

About 4 years ago, air tubes leading to injectors and ICV were replaced, along with fuel pumps, fuel accumulator, fuel valves (attached to the pumps), and injectors, and the black vacuum valve near the ICV. Fuel filter was replaced about 3 years ago, at 242,000 KM.

All vacuum tubes and rubber connectors in the engine compartment were replaced about 5 years ago.

[Xsbank]

Do you have a spark? At the coil lead? At the plugs?
Is it getting fuel?
Can you warm the garage up? Ice, water in the fuel?

High idle might be the coolant sensor that keeps the idle up until it warms up.
Battery voltage? Does it turn over fast?

Gotta be something simple if it just happened.

[Robert Squires]

* CH, you're actually better off now than before when you just had the idle rpm problem. This should be pretty straightforward. Xsbank's advise is spot on. Find out whether you're missing spark or missing fuel.
* Given all the parts and serevice you've done to this car, when you've got the engine running again, and you're back to the high idle problem, you might want to check the oil temp switch (on the block, next to the oil filter). It's responsible for raising the idle rpm when the engine (oil) is cold (colder than the switch point).

[Cigar Havana]

Charged battery 4 hours at 2 amps. Checked battery voltage, it is 12.82 volts after 5 min. off the charger. Cracked open the fuel line at the cold start injector, fuel poured out, ignition is off. Disconnected spark plug wire 8, connected it to a spark plug and grounded the plug threads, cranked engine and saw blue sparks.

Temperature is now down to -13c, no chance of heating garage.

Checked resistance on that blue 2 pin temperature sensor on the top pipe leading to radiator. 21.82 kohms between pins.

Fuel dististributor was replaced with a rebuilt one about 3 years ago.

Next steps?

[Cigar Havana]

After sitting 12 hours at -12c, battery reads 12.7 volts. Battery became fully charged in 20 minutes. Then I tried 3 long cranks, it still won't start. Pulled 7 plugs and they all looked black and a little wet. I guess the cylinders are getting fuel.

Could it be a weak coil? Coil was replaced about 4 years ago?

Could some wire be broken that causes a no start but allows cranking?

Come to think of it, after replacing the ICV, cap and rotor, it started a little rougher.

Any recommendations? I guess it will have to go to the shop tomorrow.

[Xsbank]

Have another look for a spark. If you had one, you'd likely be getting something. Any exhaust smoke? Got a spark from the coil lead?

[wbain5280]

Make sure your cold start injector is firing when cranking. Remove the blue connector at the csv, connect a trouble light to the non brown wire, which in MB speak is the ground, and crank the engine. It should light up and go out.

I had the same problem with my 420SEL and it's a problem with my fuel pump relay. Since I'm a cheap SOB, I decided to bypass the FPS program and added a diode from the start pin to the injector pin. The car starts but doesn't run very well, yet. It's 20F now in my area.

[pmckechnie]

It takes AIR, FUEL,COMPRESSION,IGNITION (at the correct time),and EXHAUST. One or more is missing. Too little air or too much because of big vacuum leaks. Not enough fuel because of stuck air sensor plate or pluged cold start injector or possibly no power to the cold start injector. Compression low. Ignition problems because of gas fouled plugs which need to be replaced. And finally, the exhaust pluged up by varments (I have seen it happen).
Most cars I work on that won't start are sent to me because of something the owner or other shop doesn't understand. They say it must be a computer problem so send it to Paul. Most could have been fixed by just checking the 5 things above and fixing the ones that are bad.

[Cigar Havana]

Cleaned both terminals and all lugs on the coil. There was corrosion on the driver's side terminal. I also cleaned all ground wires beside the coil. Put the pedal to the metal, turned ignition to start, and held it there for about 5 seconds, until it fired, let the pedal up as soon as it fired. There was a huge amount of white smoke out the exhaust pipe for about 1 min., ran rough for about 3 min., purred like the pussy I know until turned off.

Basically, there was water in the fuel. Looking back over my diagnosis, I guessed there was water in the fuel because the wet plugs did not smell like gasoline

[Xsbank]

I'm glad that you got 'er running, but if you have that much water that the plugs are wet with water, you have a blown headgasket. I'll bet that the continuous cranking with the throttle wide open blew out enough water that you could start it, but the "white smoke" was the engine getting rid of the coolant that had leaked in. With the car running, look for bubbles in the coolant, or cloudy coolant, or a dropped level in the coolant.

[Cigar Havana]

How can I confirm for sure the head gasket is gone? Leave it for another week and see if it fills with water/coolant again?

The coolant was down about 1/4" but I thought was due to the cold temperature. The coolant is clear in the plastic tank.

I did smell the exhaust and there was no antifreeze smell. That big cloud of white smoke reminded me of a Lincoln that blew a head gasket and you could smell the antifreeze in the exhaust.

[Robert Squires]

* So it looks like the overriding problem was water in the fuel(tank), coupled with a weak/intermittant spark due to poor connection(s) in the ignition primary circuit.
* Are you planning to drain the tank? If you have enough water to kill ignition (the engine cranked but didn't hit at all), a moisture additive might not be enough. Using a jumper at the socket for the fuel pump relay (to run the pumps w/o running the engine) you can empty the tank from the fuel inlet hose under the hood (disconnect the hose and connect it to a suitable drain into a refuse oil/fuel can). I think your car still had a stand-alone fuel pump relay (before MB got rid of the relay and incorporated that function into a control unit).
* Water through the combustion chamber, in my experience, doesn't automatically result in a damaged head gasket. Just add clean fuel (a fuel filter would be nice too), put in fresh plugs, and drive it. You'll know soon enough.