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Old 03-15-2000, 10:17 PM
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Hi, I just got the service records for my 88 300E (I've had it for about two months) and I have a few questions I was hoping you could help me with. The car has 197,350 miles and the idle is a little rough. The car is fine while under acceleration but come to a stop (after it's warm) and it shakes. The rpms go up and down a bit and the oil pressure drops a bit below 1. Apparently the previous owner (who bought the car with 64,500 miles in 10/93) had this problem before and was never fully resolved. My questions are the following:
1) The last tune-up was done on 12/97 at 162,500 miles. Could the rough idle be helped by a tune-up?
2) On that date the prev. owner took it in for engine noise. The shop wrote on the repair order that there is "a little lifter noise. Early signs of valve job. Blue smoke with rubber smell after car sat for three days." What's involved in a valve job? The car never had the valve job and I don't notice any noise. Should I be worried about that?
3) On 6/98 (170,000) rough idle. A scope test was performed and checked OK. What is that and what can it tell you about the idle.
4) On 8/98 (171,100 miles) the owner took it in because the car was idling rough and would occasionally die. The shop wrote "rust in fuel head. further repair might be necessary." What is that? It was never repaired. Should I have a tech look into it?
Thanks. I'm sorry for the long post but this is my first Benz and I don't know much about them (yet).

[This message has been edited by arias (edited 03-15-2000).]

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Old 03-15-2000, 11:01 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
The valve job will take the shop a few days and is under $2,000 in most cases. The rust in the fuel head, in my feelings is usually misdiagnosed. I have seen people take the lines loose and some rust comes out. This is normal for the fuel is taking out the rust from between the line and the nut. Don't worry about this one.

1981 280GE SWB
1987 16V

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Old 03-15-2000, 11:21 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Mind you I'm going to tell this from a techs viewpoint.

#1 What the H is a tune-up. Your car tunes every known adjustment multiple times a second. If the last time plugs were replaced was 35000mi ago well ... DUH.

#2 You will need a valve job when your valves seal so poorly that you have a compression loss. Since quantity of leakage is a time dependent issue, valve leakage hurts effective compression more at an idle speed as there is more time to leak between cycles. Have a compression test and/or cylinder leakage test done when you put them plugs in.

#3 A scope test can mean many things. In this frame of reference it is being used to monitor primary and secondary ignition. An oscilliscope is a devise that plots voltage against time. A proper spark event will have a given signature describing each stage of the ignition cycle. An activation voltage of around 10,000 volts is necessary to ionize the air gap in the plug till the spark can pass through the gap (at idle things doing well). During the actual spark the conditions in the cylinder can be read (if one is good and works on the same systems daily)lean, rich, timing?, compression (not directly here although a relative compression test can be done on good scopes by measuring the starter current keyed to engine firing order). Mainly one can tell deviations from cylinder to cylinder and see a glimse in a rather rapid event. A good spark will take a minimum of one millisecond to finish its event, probably no more than 2 milliseconds at the most. Events less than 1 millisecond are probably in misfire. From here it gets technical (bg).

#4 Forget you herd dat. Replace the fuel filter and pray.

Well after you verify your compression and do a cylinder balance test to identify the weak or misfiring cylinder, the next step involves determining the weakness that causes the misfire. A cylinder balance test monitoring hydrocarbons will identify cylinders recieving more or less fuel. The concept here is that by killing the ignition on a specific cylinder and watching how much unburned gas appears in the exhaust with an exhaust gas analyser you get an idea of the relative amounts that were put in. This test is done by the machine under precisely timed conditions or it would be impossible.
Once you have determined that there is good reason to suspect fuel flow difference (ooh that rust ain't no good here) it can be proven with a differential flow meter which a few dealers and most BSCs (Bosch Service Center required equiptment) have. We test at 8 cc/min for idle 30-40 for midrange and around 60 for full throttle. The idea is to measure all at a flow rate in the range and the variation cylinder to cylinder can't be more than ten percent. The actual flow value isn't the consideration just a position to compare. This test can be done with just the fuel distributor or also with the injectors to precisely identify that $700 bad fuel dist.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[This message has been edited by stevebfl (edited 03-15-2000).]

[This message has been edited by stevebfl (edited 03-15-2000).]
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Old 03-16-2000, 07:28 AM
Posts: n/a
You bought a car that hadnt been serviced for 30k?

Get a service first ...........most probs should be resolved.

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