'81 380sl major vacuum leaks.. please help

[Chris_Ginsburg]

1981 380SL

I've been trying to figure out what's wrong with this car for 5 months! Hard starting when cold (15-second cranks to start). Then idles OK until the cold start valve turns off... after that - surges / misfires. Feels like complete fuel pressure collapse if the pedal is depressed too quickly. -Stalls often. Misfires/ surges or "ticks" every 20 seconds. Runs/idles OK after about 2 hours of driving. Outside temperature also seems to have some affect.

I replaced the entire fuel pump/pressure system in the rear. Checked fuses. I replaced both the idle contol unit and the idle control valve.

Finally smoked the injection system and found leaks at the injectors - so mechanic replaced the injector seals (actually he replaced the injector holder o-rings - same thing?)

Didn't help!

I just sprayed carb cleaner in the injector holes and it changes the idle (wants to stall).

The injector seals were just replaced! Where else could a leak come from at the injector? It seems like all the injectors react to the carb cleaner spray somewhat. Some react more than others.

Is there a difference between injector seals and holder o-rings? Can the injectors themselves all be leaky?

This is driving me crazy because there must be a massive vacuum leak. not some pinhole. The car runs terribly and I'm afraid I'm going to brake my single-timing chain! Any help is VERY appreciated.

Thanks,
-Chris

[ctaylor738]

The injectors sit it a jacket that is sealed with an o-ring. There is also a seal between the injector and the jacket. Take a look at any of the online parts catalogs and you will see what I mean. Not to come between a man and his mechanic, but it makes no sense to replace just the o-rings.

Can you run a check of system and control pressures and get a reading of your on-off ratio? Lots of posts on how to do this. Also, look at a post by Timm9 about his troubles with a cracked air guide housing,

[Chris_Ginsburg]

Thanks for that C taylor..

I'm really ticked off that my mechanic replaced just the o-rings. They did ask me if I wanted the injectors replaced and I said no because I was focused on air leakage...but they should have f'ing looked at the injectors. I told them I was trying to plug up a vacuum leak. Of course the O-rings looked new when they took them out and that didn't ring any alarms. $300 wasted and now onto another mechanic.

I've dropped over $1K on this issue...and still not resolved. You'd think there would be a decent vintage Merc mechanic in San Francisco. I'm about to just give up and coat a mound of hi-temp silicone around each injector. I wonder if that would plug up the leaks? I think it could be removed later if someone (else) wanted to pull out the injectors again? Ghetto, I know but I'm done wasting money.

Anyway, thanks again.

[ctaylor738]

Seems to me that you ought to be able to work something out with the shop to do the injector seals. That is so basic that they ought to own up to their mistake. You paid them to fix vacuum leaks around the injectors and they replaced the wrong parts. It's only a couple hours of labor and $40 in parts so it ought to be a resolvable issue.

But this sounds like more than injector seal leak. It sounds like for whatever reason, the fuel mixture is 'way lean.

First, your statement about the cold start valve makes no sense. In your climate, the cold start valve probably doesn't fire at all, at most it would just be a tiny puff and would not play in the problems you are having.

There are several warm-up related switches in your system.

1. a water temp sensor on the back of the head or intake (right/pass side) that raises the idle speed
2. an oil temp sensor on the oil filter housing that kicks the lambda system into closed loop which means the brain is trying to control the mixture based on input from the oxygen sensor
3. one or two temp-controlled vacuum switches on the intake manifold that control how enrichment is handled at different engine temps and open vacuum to the purge valve.

If I were you, I would focus on 2 first. It sounds like you are having mixture problems when the system goes to closed loop. Pull the wire off the oil temp sensor and ground it with a jumper wire. (Make sure you don't disconnect the oil pressure sender, which is also down there.) This will keep the system in open loop. If things improve then you at least know where (part of) the problem is.

Also -

Make sure that all of the vacuum plumbing (switches, lines, connectors) is tight. Test that the the warm-up regulator holds vacuum on both sides.

Make sure that the purge valve (left side front) is connected correctly and not leaking.

If none of this helps, you can try adjusting the basic mixture setting. You stick a long-handled 3 mm hex key down through the hole in the air cleaner. Turn it gently until you feel it drop into the screw and then push it down to engage the adjuster. Turn it to the right (clockwise) to richen. Don't go more than 1/8 turn at a time, a max of 1/2 turn. Be sure to keep track of what you've done so you can get back to where you started if nothing changes.

[meltedpanda]

This may help
Idle Bad with Cold Engine K jet system
1. Leaks in air intake system:
Check all air intake and vacuum connections. Any old hoses must be replaced.
Be sure to check the intake boots around the intake runners. Check all hose clamps.
Replace any collapsed hoses.
2. Auxiliary Air Valve does not open:
Check the hose connections to the valve. If these check good remove the valve and
check for leaks by closing off one end and applying a small amount of suction to the
other end, if air can be drawn through the valve, with the other end capped off,
replace the valve.
3. Cold Start Valve leaking:
Remove the cold start valve, located in front of the intake. Leave the fuel line connected,
place the valve over a container, disconnect the electrical connection, apply 12 volts
to one of the connections, apply a ground to the other side, look for an even conical
spray pattern, turn the power off, wait a few seconds, there should be no leaks. If
the pattern is not correct or the valve leaks, replace the valve.
4. Cold control pressure not correct:
Check the warm up regulator. I would look for a pressure of at least 1.25 -1.50 bar
(18-22 PSI) at freezing temperatures. Be sure to check the pressure coming from the
mixture control unit.
If the pressure is not correct, do a complete check of the warm up regulator.
5. Injector Leaking:
The opening pressure could be to low, opening pressure should be around 2.5-3.6 bar
(36-52 PSI). Check all the injectors for evidence of leakage. Test for opening pressure.
When you reinstall the injectors be sure to replace the o-rings.
6. Injected Quantities low:
Compare the fuel quantities of each injector. Place each injector over a container and
record the amount of fuel each produces at a given amount of time. They should be equal.
7. Basic Idle incorrect:
Check and rest the idle per the specifications. It is best to set it when the engine is warm.
Check the co output, also.
8. Fuel Leaks:
Inspect all fuel connections.
9. Fuel injector Clogged:
Pull the injectors and inspect for correct operation. Apply power to the fuel pump,
jump the wire terminal numbers 30 and 87a. Caution fuel will flow from the injectors
when the air flow plate is moved in an upward direction. Check for correct operation.
10. Control Plunger in the fuel distributor binding:
Remove the air cleaner and carefully move the plate up and down, do not force the plate.
If it binds investigate. If it is not simple to repair replace the unit.
11. Idle speed, Ignition Timing, and mixture out of tolerance:
Check and adjust all of the above.
12. Faulty Ignition System:
Check the ignition system. 1973-1977 check point gap .016, correct plugs see chart,
Condition of cap and rotor, 1978-1983 check for a distinctive hum in the CDI with ignition
on in engine compartment, (if no hum the CDI may be defective) CDI is the ignition
module located on the relay panel in the engine compartment left side.