1991 300CE Hard Start

[glenmore]

1991 300CE, 179m.

I have owned this car since 6/2002, 111m.

The start has always been a little hesitant. With a cold start, on occasion it will fire up immediately, (like a modern EFI car), but most of the time it takes a few beats. It has gotten progressively worse and now takes 2 tries to start. Hot starts after a few minutes are OK but anything longer results in a difficult start. The car will lug a bit before smoothing out.

Battery a bit old, changed at 11/2003, 125m.

Cap and rotor last changed 10/2006, 141m.

Fuel accumulator changed at 3/2007, 143m, with no change to start.

New plugs, old ones all look pretty good, 11/2009, 160m.

Other than the start, car drives and idles perfectly.

Because the car drives normally, what can you eliminate as suspect with a hard start?

Thanks,

glenmore
1991 300CE
1999, 2000 C280
2000 C230K

[Ferdman]

glenmore, a faulty fuel pump check valve may allow fuel pressure to decay when the car sits for an extended period resulting in hard starts. I would check the condition of the distributor cap and rotor ... remove any deposits, or consider replacing both. Amazed that the battery has lasted that long ... most MB batteries last only 4 years or so. When were the ignition wires and Oxygen sensor replaced last? Normally best to replace the Oxygen sensor every 60,000 miles.

[glenmore]

Thank you for the suggestions.

O2 sensor was changed 9/2007, 146m.

I have never changed the wires.

Last time I checked the cap and rotor, there was a little bit of corrosion there. Also a few cracks at the mounting screws.

Best,

glenmore

[mbzman]

Have you ever replaced the injectors? If they are original they are likely due for replacement. Does the early M104 use mechanical or electronic injectors? If they are mechanical then they fairly inexpensive to change out. Old injectors will bleed down after engine is turned off resulting in the hard starts.

[glenmore]

Injectors/seals/insulators were changed 4/2007, 144m. No change to the start. These injectors are the mechanical kind.

[ps2cho]

Clean the corrosion off the rotor. Usually I don't advocate fuel pressure tests because usually it provides no information. If the car starts, usually pressure is fine, if it doesn't pressure is not (other factors removed), but in this instance knowing if the system can hold the 2.8 bar is important and it should tell you if its fuel related or not.

Easy check would be engine temp sensor. Make sure its within spec. ~250ohms hot, 2000-3000 ohms cold depending on how cold it really is.

I have a AAA battery in my wagon...going on 5 years soon. Knock on wood!

[dubadaddy]

I had the same problem with approx same mileage on my 1992 300CE and turned out to be the EZL. Replaced and no more problems. Sometimes you can find them on Ebay but you have to cross-reference your cars VIN number with the numbers on the EZL. The problem is it can be dicey to buy an electrical part on line and so I went to the dealer. Cost at the dealer is about $1200 with my Mercedes Benz membership discount.

I happen to have it after the car crashed. It has about <10K on it and have the paperwork to prove it. Only if you are interested.

[S-Class Guru]

Glenmore, I assume your engine is a 104 with the CIS injection system?
I have looked for a similar problem on my '91 SE for only 18 years or so. I've checked and replaced every part on it, with the exception of the complete fuel distributor. Still looking.
It's not too bad, always starts and runs perfect on the 2nd try and a few seconds of chug-chug. But it's slowly getting worse.
The usual suspects are as mentioned; fuel pressure loss due to check valve, regulator, accumulater, injector. Weak ignition due to wires, plugs, dist, rotor, coil.

None of these, or a thousand other things, helped mine.
I'm still waiting for that "That's it" moment.

The things that had at least a little temporary improvement were the two sets of new injectors, and adjusting the EHA a bit richer.

I could write a book on the lack of success and theories I've tried.

DG

[mglawoffice]

overvoltage relay defective( or fuse in it blown) will cause hard cold start

[Guest987]

What you have to eliminate to solve a hard start issue?? Well, for WFWIW my approach to solving a like situation with a 190e 2.6. First, you have to know what is involved with the start-up of the car. Studying the circuitry and knowing exactly how it works. Once, you do that, then you can start to eliminate the items to find the fault. It's a task, but with determination you can do it without a lot of tools or investment. I saw where people in some threads I read had replaced nearly two sheets of paper showing replaced items, and it still didn't fix the problem. I failed to fall prey to such a loss of money that way. I did replace the ovp with a new one with 2 fuses. That wasn't the problem however. The big issue I find with the old mercedes even is they are very unforgiving to out of tolerance information sent to the cpu. That can be from replacement parts that were not oem quality. I have now went for 3 months with no fail starts in 1inch or more rain and now 19degree weather outside. You will probably find something, that is a resistive fault causing a lock-out.

[ohiomike]

Glenmore, I am the person who sold you the Euro Lights for that car. When I had a 300CE I always changed the spark plugs first and if the Oxygen Sensor has 30,000 miles on it, I would change anyway. The O2 sensor never made the Dash light up but changing it always made the car run better and stopped the spark plugs from fouling dur to rich feed of fuel. It also made the catalytic converter stop from burning out due to improper burning of fuel. Very nice car though!