K-jet 450 SE cold/hot start issues - PeachParts Mercedes-Benz Forum

116fan · #1 02-19-2012, 12:56 AM

G'day y'all,

My car is an early '76 K-jet with 308 000kms, which I've recently had recommissioned after 8 yrs in storage.
New fuel tank, fuel pump, accumalator, injectors, lines, replaced fuel distributor with known good one, warm-up regulator tested OK, fuel pressures good, all new filters, vacuum lines etc etc.
The motor, once fully warmed up, runs beautifully smooth - it simply cannot run smoother - so that's all good.
Problem is when starting up cold - during winter it required quite a bit of cranking and spluttering to get going. I'd have to let it idle for a few minutes before stepping on the gas, otherwise it'd splutter and I wont be able to accelarate.
As the season changed to summer, cold starting became easier to the point where I now only have to crank the starter for around 3-5secs during which time the motor would gradually come into life.
It would always start idling at @ 650rpms and then gradually move upto 1000rpms when cold and then back down to 750 rpm as the motor warms up.
Is this the CSV or thermo time switch possiby faulty?

Hot starts are fine if restarted within 5mins, in other words I don't have to step on the gas when restarting.
Longer than 5 mins the car would still start-up with barely a crank from the starter, but it would idle at 600 rpms in N and then 500 rpms in D.
After sitting for 30mins or longer, the motor would fire up with one crank of the starter but fails to idle. As soon as it fire's up it dies.
However, if I step on the gas while starting it would fire up smoothly and idle, but at 500 - 600 rpms, and then after quite some time get back upto 750rpms.
Is this a faulty AAV?

Any ideas appreciated.

mercmad6.3 · #2 02-19-2012, 03:24 AM

You are losing pressure somewhere by the sounds of your symptoms. First suspect would be the pressure valve in the fuel distributor ,next would be an accumulator or the non return valve in the pump.

meltedpanda · #3 02-19-2012, 03:07 PM

on the Kjet the AAV stuck closed or nearly closed would cause a hard cold start or non start . If you have to keep your foot on accelerator to get it to run cold this may be your issue.
It would also cause a rough cold idle
besides fuel pressure as mentioned, leaky cold start injector or fualty injectors could also be a cause
Oh ..and make sure yopur airflow sensor/plate is operating smoothly

116fan · #4 02-20-2012, 02:01 AM

Thanx for the advice guys - yes it does idle a bit rough for the first minute when cold, but then smooths out.
I'll get the pressure valve in the FD checked out.
I'll check the airflow/sensor plate as well.
Like I said, new injecotrs from Mercedes... but you never know.

gingerkid · #5 12-06-2012, 12:13 PM

116fan any solution to this? My '79 350se started doing this after redoing all of my vacuum/air hoses. Old ones were brittle and leaky, this might have exposed an issue with my AAV? Mine starts up immediately and fine but the first min of idling it chugs. Throughout the rest of the day when it is cold it starts and idles fine, just the first start in the morning.

meltedpanda · #6 12-06-2012, 02:41 PM

the AAV should be open when cold, closed when hot, if you have shored up vacuum leaks then it may in fact indicate an AAV issue

mercmad6.3 · #7 12-06-2012, 04:09 PM

Quote:

Originally Posted by gingerkid

116fan any solution to this? My '79 350se started doing this after redoing all of my vacuum/air hoses. Old ones were brittle and leaky, this might have exposed an issue with my AAV? Mine starts up immediately and fine but the first min of idling it chugs. Throughout the rest of the day when it is cold it starts and idles fine, just the first start in the morning.

you have an internal pressure leak somewhere,possibly at the pump . The Djeet pump has a small non return valve i the outlet side and if it's worn,nitial line pressure drops overnight. The system has to come back up to pressure when you start ,first time and it is what causes the chugging etc.

meltedpanda · #8 12-06-2012, 07:38 PM

Quote:

Originally Posted by mercmad6.3

you have an internal pressure leak somewhere,possibly at the pump . The Djeet pump has a small non return valve i the outlet side and if it's worn,nitial line pressure drops overnight. The system has to come back up to pressure when you start ,first time and it is what causes the chugging etc.

respectively disagree on that, the pump should spin up to pressure when the key is in "2 " position, pressurize the system at that time.

mercmad6.3 · #9 12-06-2012, 08:23 PM

Yes, that is correct but if you notice,the pump only runs for a couple of seconds,and on these cars the system is 40+ years old .Old components have little quirks which can trap the unwary.

.So, if the pressure has dropped off to zero,then it isn't sufficient to pump up again to the 28 PSi. It can be overcome by simply turning the key to the on position from Off a few times to get the pressure up .
Another source of pressure leakage is the cold start valve and the injectors themselves . All cause raw fuel to dumped into the engine as it sits . Often a dose of injector cleaner fixes this.

meltedpanda · #10 12-06-2012, 08:50 PM

agree.. after varifying that the running pressure is correct. CSV and Injectors are a good place to start looking if the pressure drops overnight .

Does the oil smell like gas?

gingerkid · #11 12-06-2012, 09:55 PM

wow thanks guys, this forum always pleasantly surprises me with the amount of knowledge and willingness to help. It is a K-Jet, my 73 280se has djet, it is really neat to see the advancements MB made from 73 to 79! Exciting stuff! And 28psi... i assume we are talking about fuel pressure?

A few things I will be checking out...

-"small non return valve i the outlet side." if the kjet has the same thing.

-I will try turning the key to off and on positions a few times and see if that helps. Though I do always make sure i wait until i hear the pump turn off after its initial pump when i get to position two, before i try to start the car.

-i have receipts for new injectors and seals from less than 30,000k miles ago but that was 10-12 years ago. One owner car with all the receipts!! the car did go through a period where it was barely driven for about a year and then recently sat for 2 months just being started occasionally.

-I put a can on Seafoam in the gas tank recently. I was also fortunate enough to find a gas station with ethanol free gas so i've been running that.

-I will check the oil for gas smell.

gingerkid · #12 12-07-2012, 09:17 AM

OK so this morning i put the key in to position 2 took key out, repeat a few times, and maybe it was because it was early but i swear i didnt hear the pump run?

Also I want to double check the vacuum line that goes from the rear breather hose to what appeared to be the FRP, maybe the damper, I dont recall. Anyways it was hard to reach and I'll want to make sure it is snug.

gingerkid · #13 12-07-2012, 01:18 PM

Ah so more research on the k-jet system.. what looks like the FPR is the fuel accumulator: "When the engine is stopped and all of the non-return valves close, the spring pressure against the diaphragm will maintain the residual or holding pressure and overcome any slight seepage."

gingerkid · #14 12-07-2012, 02:35 PM

another notable issue before my vacuum fix-up was the line that went from the warm up regulator to the hose between the manifold-AAV, was leaking. So i wonder if the warm up regulator could be the culprit? So many possibilities!