Temporary rough idle warm & cold after starting - Mercedes-Benz Forum
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post #1 of 60 (permalink) Old 05-23-2019, 08:53 AM Thread Starter
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Date registered: Apr 2019
Vehicle: 1990 420SEL
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Temporary rough idle warm & cold after starting

Have searched forums extensively here and elsewhere in attempting to troubleshoot issue. 420SEL starts immediately, but idles really rough for approximately 10 seconds before smoothing out. Behaves the same whether it's started after sitting overnight or after warmed up and then re-started after 10 minutes or so. Based on fuel accumulator reads it doesn't appear to match this scenario and no vacuum leaks were detected based on gauge.

I've replaced cap/wires/rotor (originals with 87k m) and put in non-resistor NGK's. This improved idle which is smooth when warm although occasionally I also have a slight rough idle only when in gear. Engine under load runs exceptionally smooth with no indication of miss or roughness felt.

Possible related issues - in D or R engine idles at 500 rpms (seems low) also I have a slight, but consistent tick coming from passenger side valve cover leading me to think I may have a rocker arm, shim or accumulator issue. Not sure if there is any way to check clearance on rocker arm without the special tool.

Thoughts on what to check first for temporary rough idle?
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post #2 of 60 (permalink) Old 05-23-2019, 10:41 AM
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Have searched forums extensively here and elsewhere in attempting to troubleshoot issue. 420SEL starts immediately, but idles really rough for approximately 10 seconds before smoothing out. Behaves the same whether it's started after sitting overnight or after warmed up and then re-started after 10 minutes or so. Based on fuel accumulator reads it doesn't appear to match this scenario and no vacuum leaks were detected based on gauge.

I've replaced cap/wires/rotor (originals with 87k m) and put in non-resistor NGK's. This improved idle which is smooth when warm although occasionally I also have a slight rough idle only when in gear. Engine under load runs exceptionally smooth with no indication of miss or roughness felt.

Possible related issues - in D or R engine idles at 500 rpms (seems low) also I have a slight, but consistent tick coming from passenger side valve cover leading me to think I may have a rocker arm, shim or accumulator issue. Not sure if there is any way to check clearance on rocker arm without the special tool.

Thoughts on what to check first for temporary rough idle?
I would do a fuel pressure leakdown test with a gauge. I'm thinking one or more injectors are leaking after the engine turns off, or the seal at the bottom of the control piston in the fuel distributor is leaking. Check the latter with a strong light and mirror to look under the air flow meter flap for puddling fuel.

You might also disconnect your cold start injector when the engine is warm and see if the problem persists under that condition.
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post #3 of 60 (permalink) Old 05-23-2019, 12:26 PM Thread Starter
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Thx lucky Mike, will use mirror and light while holding plenum down to look for leaks. Would imagine fuel could be smelled if leaking when plenum held down as well?

I have ordered manual discs online for future use, but would like to check fuel pressure with my fuel gauge/attachments prior to discs arriving.

Not sure where to hook up gauge and/or if this also requires a special MB fuel pressure gauge to check?
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post #4 of 60 (permalink) Old 05-23-2019, 06:04 PM Thread Starter
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Picked up a mirror, but don't think it will be necessary after all. Pushed plenum down and I can hear fuel dripping inside and can also certainly smell it. What does this mean? I am pretty certain fuel isn't supposed to be dripping inside here.
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post #5 of 60 (permalink) Old 05-23-2019, 11:33 PM
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Picked up a mirror, but don't think it will be necessary after all. Pushed plenum down and I can hear fuel dripping inside and can also certainly smell it. What does this mean? I am pretty certain fuel isn't supposed to be dripping inside here.
It likely means the control piston seal has failed. It's a straightforward replacement but follow the manual exactly so you don't introduce variations in the function of the fuel distributor. When you get your manual it's section 073-207.

I would do this first: With the car cold, prime the pumps two or three times by turning the key to run but not starting the engine. Then push down lightly on the air flow meter flap and see if you've got about 2mm of play before you hit resistance. Mind you, I am NOT an expert by any means. This is just stuff I've read in the manual. I advise people to do their own reading.

Anyway, I'd do the same check with the engine warm. In both cases, you might find you have to use a magnet to lift the flap up to the top of its travel. The main thing is there needs to be a little free play before the flap mechanism touches and starts moving the control piston.

If this all checks out, I would then check the car in the morning after it had sat all night. Push down on the flap and look for fuel in there. You won't get any resistance this time because the fuel pressure has bled down overnight. If the control piston seal is leaking overnight, you'll see fuel in there.

You can still buy the seal at the dealer.

Man, I wish H.D. would chime in. I guess he will if I make any horribly way off recommendations.
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post #6 of 60 (permalink) Old 05-24-2019, 08:31 AM Thread Starter
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Appreciate the advice! Now it makes sense as to why my oil level is too high, explains stumble and rough idle as well.

After reviewing various threads and instructions on rebuilding Fuel Distributor I have decided to go with a reman FD from CIS Flowtech out of Alabama. Likely won't arrive for a couple of weeks so in the meantime I'll look into changing out injector O-rings etc.
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post #7 of 60 (permalink) Old 05-24-2019, 08:41 AM
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Appreciate the advice! Now it makes sense as to why my oil level is too high, explains stumble and rough idle as well.

After reviewing various threads and instructions on rebuilding Fuel Distributor I have decided to go with a reman FD from CIS Flowtech out of Alabama. Likely won't arrive for a couple of weeks so in the meantime I'll look into changing out injector O-rings etc.
You're welcome.

Don't be scared of changing that control piston seal. It's not that big of a deal and is nothing like doing a rebuild. Of course, if you have the resources, it's great to start with a fresh fuel distributor. Just saying, don't be put off by getting into it a little.
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post #8 of 60 (permalink) Old 06-13-2019, 10:01 PM Thread Starter
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Update - I have decided to opt for a fuel distributor rebuild and fuel injector cleaning svc/test via Ebay. Today I removed fuel distributor and injectors on 90 420SEl with 87k miles for reference. Car is original Seattle bought life long resident so you can imagine based on our climate that it's friendly to rubber, interiors and not prone to rust. In fact car is completely rust free and many of the rubber items like axle boots are in incredibly good shape.

Anyhow upon removing fuel injection items here is what I found interesting - job was pretty easy (one fuel connection bracket had to be slightly and temporarily moved via pliers to remove 1 fuel injector holder allen bolt) with total time to remove fuel distributor, injectors and plastic nozzle holders about 70 minutes. No injector felt secure in it's plastic nozzle holder after removing clamps. I understand they should have still been secure in their fitment. This tells me I probably had one or more injector that wasn't completely sealed and thus letting in unregulated air. Some of the plastic nozzles were also loose in intake manifold and could be spun freely which would cause potential additional air leak issues. After removing I certainly wouldn't trust a 30 yr old car with original injector nozzles and fuel injector seals to be sealing properly so I ordered all knew ones this evening as they are very inexpensive.

Boxed up fuel distributor and injectors to send for service and will update in a few weeks on outcome.


(For those following my other thread on differential I'm still trying to figure out best option, used replacement or rebuild Will update on that thread upon conclusion. Diff comes out this weekend, so much for low key and relaxing on Fathers Day).
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post #9 of 60 (permalink) Old 06-13-2019, 10:11 PM
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Originally Posted by JR420 View Post
Update - I have decided to opt for a fuel distributor rebuild and fuel injector cleaning svc/test via Ebay. Today I removed fuel distributor and injectors on 90 420SEl with 87k miles for reference. Car is original Seattle bought life long resident so you can imagine based on our climate that it's friendly to rubber, interiors and not prone to rust. In fact car is completely rust free and many of the rubber items like axle boots are in incredibly good shape.

Anyhow upon removing fuel injection items here is what I found interesting - job was pretty easy (one fuel connection bracket had to be slightly and temporarily moved via pliers to remove 1 fuel injector holder allen bolt) with total time to remove fuel distributor, injectors and plastic nozzle holders about 70 minutes. No injector felt secure in it's plastic nozzle holder after removing clamps. I understand they should have still been secure in their fitment. This tells me I probably had one or more injector that wasn't completely sealed and thus letting in unregulated air. Some of the plastic nozzles were also loose in intake manifold and could be spun freely which would cause potential additional air leak issues. After removing I certainly wouldn't trust a 30 yr old car with original injector nozzles and fuel injector seals to be sealing properly so I ordered all knew ones this evening as they are very inexpensive.

Boxed up fuel distributor and injectors to send for service and will update in a few weeks on outcome.


(For those following my other thread on differential I'm still trying to figure out best option, used replacement or rebuild Will update on that thread upon conclusion. Diff comes out this weekend, so much for low key and relaxing on Fathers Day).
I've only done the injectors and cups once but I thought they were kind of loose after replacement, too. I think the new orings afford that movement. I did find some vac leaks before the project by spraying carb cleaner or something around the injectors.

Looking forward to your report on the diff. I just had the car in the air this week and noticed what could be lube coming out of the left side at the axle.
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post #10 of 60 (permalink) Old 06-14-2019, 07:25 AM
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Originally Posted by JR420 View Post
Update - I have decided to opt for a fuel distributor rebuild and fuel injector cleaning svc/test via Ebay. Today I removed fuel distributor and injectors on 90 420SEl with 87k miles for reference. Car is original Seattle bought life long resident so you can imagine based on our climate that it's friendly to rubber, interiors and not prone to rust. In fact car is completely rust free and many of the rubber items like axle boots are in incredibly good shape.

Anyhow upon removing fuel injection items here is what I found interesting - job was pretty easy (one fuel connection bracket had to be slightly and temporarily moved via pliers to remove 1 fuel injector holder allen bolt) with total time to remove fuel distributor, injectors and plastic nozzle holders about 70 minutes. No injector felt secure in it's plastic nozzle holder after removing clamps. I understand they should have still been secure in their fitment. This tells me I probably had one or more injector that wasn't completely sealed and thus letting in unregulated air. Some of the plastic nozzles were also loose in intake manifold and could be spun freely which would cause potential additional air leak issues. After removing I certainly wouldn't trust a 30 yr old car with original injector nozzles and fuel injector seals to be sealing properly so I ordered all knew ones this evening as they are very inexpensive.

Boxed up fuel distributor and injectors to send for service and will update in a few weeks on outcome.


(For those following my other thread on differential I'm still trying to figure out best option, used replacement or rebuild Will update on that thread upon conclusion. Diff comes out this weekend, so much for low key and relaxing on Fathers Day).
my distributor/injectors arrived in California (to the same rebuilder) this past Wednesday. Havent heard back from him yet, but i hope to have everything back by late next week. My only concern is potentially having to make various adjustments in the fuel system to get things running smoothly? Or is it possible that it will be as simple as carefully reinstalling the injectors/fuel distributor and she'll fire right up and run flawlessly?
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