Author Topic: Not another hot start problem! (Read 6542 times)

col320ce · « **on:** March 03, 2021, 08:44:06 »

Looking for advice on the warm start issue with the 250SE (same engine and set-up as the SL so I figured it was more useful here).
Basically it won't start at all when hot no matter how hard I try (yes, tried flooring or half accelerator pedal, 10 seconds fuel pump before starting etc). However it WILL start if I disconnect the relay to the injection pump fuel enrichment solenoid. Cold start is fine.
Tests I have done:
All relays cleaned and work fine on the bench. Has the 2 rectangular relays and the cylindrical "1 second" relay.
both relays #87 female to positive - activates both FIP solenoid and CSV solenoids individually (ie I can hear it click)
Test light on pin #86 to ground - FIP solenoid - light stays on continuously. CSV stays on 1sec (warm) or longer depending on if warm or not.

As the car starts reasonably well when warm if I disconnect the FIP solenoid relay, then I assume that there is too much petrol being released BUT is it possible that the FIP solenoid could be wired with the + and - reversed? The wire connection closest to the engine is negative/ground brown wire.

Thanks for any help or ideas on trouble shooting this

mrfatboy · « **Reply #1 on:** March 03, 2021, 11:13:56 »

Assuming the SE is setup the same as an SL:

The fuel enrichment solenoid always engages when you turn the key to start whether hot or cold. It pushes the rack on the FIP forward enriching the pump only when cranking.

In my experience 🤪 when the engine is cold the rack is already pushed forward due to the WRD not in warm running mode so the fuel enrichment solenoid (when activated) has no effect. I have tested this theory on several cars by inserting the 5mm bolt in the front FIP (rack access) and activated the fuel enrichment solenoid. I have never seen any movement of the rack bolt when engine is cold.

Like you stated it seems to be getting too much fuel when trying to start hot. Have you tried disconnecting the linkage and have someone slightly crack open the throttle body intake while starting the car?

col320ce · « **Reply #2 on:** March 03, 2021, 11:36:24 »

That's a great idea... No I haven't tried it but that would help prove that it was too rich... As it's a rhd car I think I can do it myself... I'll report back tomorrow.

I can probably use the same technique with the 5mm bolt but I'm not sure where it is. If it moves the bolt when warm it means that the solenoid is wired correctly?

mrfatboy · « **Reply #3 on:** March 03, 2021, 12:02:24 »

There are some pictures of the bolt in the access hole in some posts. I just couldn’t find them now. Do some searching.

I have attached a picture of a 280sl pump. The access hole for the rack is circled. It’s usually covered with a black plastic/rubber cap. Just insert a 40mm? X 5mm bolt. Once done you can push/pull and feel the rack move. I think early pumps have a metal cap that has to be removed 🤷‍♂️

When the car is warm the bolt should move forward when starting. I usually put a reference mark on the bolt so I can see movement better. Movement is very small. I also lightly place my finger on the bolt and activate the solenoid manually and you should feel the movement.

When the car is cold and I activate the solenoid I don’t see or feel any movement. You should feel the click of the solenoid activating though.

Regarding the solenoid itself, it is adjustable. I have examined 3 different pumps. 2 of them had the same solenoid pin lengths. The 3rd pump was different but was known to be messed with. I’m assuming someone already tried to fix hot/cold starts🤷‍♂️ You can come back to that when all else fails.

Cees Klumper · « **Reply #4 on:** March 03, 2021, 13:28:48 »

Sounds like you could simply disconnect the enrichment solenoid to solve the problem. Although it would be better to check whether the car is running too rich all the time, and remediate that.

mrfatboy · « **Reply #5 on:** March 03, 2021, 17:28:03 »

I agree with Cees that your car could be running so rich that solenoid could be just putting it over the edge during starting. I just like to verify that everything is working properly and move on.

Here is a thread that has pictures. There was a reference that the bolt was 4mm but I think it was later determined it is supposed to be 5mm.

https://www.sl113.org/forums/index.php?topic=28596.0

stickandrudderman · « **Reply #6 on:** March 03, 2021, 17:44:57 »

I've seen plenty of cars that are in permanent cold start so always running rich. Have you checked the Co when hot?

wwheeler · « **Reply #7 on:** March 03, 2021, 19:39:39 »

Col,

If it makes you feel better, my 280SE does the same thing when hot. I too tried to adjust the length of the solenoid rod to limit how much it richened the mixture when hot. It is simple, with the starting solenoid disconnected, the engine starts perfectly when hot. Connected, it floods every time. I gave up and just left it disconnected. What I found though is that cold starting took longer with it disconnected. So I added the cabin CSV switch and I give it a shot before cranking. Now all starts are just about prefect every time.

I even tried the "hot start" fix from Mercedes and that only made it worse.

BTW, my WRD shuts off, CSV does not leak, spark plugs look just fine and the engine runs just fine. So I could never figure it out and chalked it up to 50+ years of wear.

Wallace

col320ce · « **Reply #8 on:** March 03, 2021, 20:09:19 »

Thanks everyone... I've got a number of things to test out today. It's been hard to tune because I couldn't restart it when hot so I'll check this.
Before doing this I'll add air as per mrfatboys suggestion at hot start to prove it's too rich.
I'll check the CO too.
BTW... Wrd is working fine. It wasn't when I got the car so it was tuned to run with the wrd still in cold start mode.

col320ce · « **Reply #9 on:** March 05, 2021, 00:03:49 »

Update...
I've leaned the fip right off (almost all the way) and I'm getting about 1000 rpm. Car is fully warm and no air sucking in on the fip wrd.
I've adjusted idle in gear to about 700.
So it's still a idling in neutral a little too high but I'll re tune it after a few more longer runs to see how it's settling.
So far it's now starting with the fip solenoid hooked up so I'm almost there.
There was also a problem with the linkages... I did the split linkage test and simply disconnecting the links was changing idle. Thought I had it perfect to start off with but it doesn't take much for these to be out.
Other weird thing is that the fip thumb adjustment is engaging in the slot but not clicking although it is moving (it does on my other car).

ja17 · « **Reply #10 on:** March 05, 2021, 02:00:11 »

As you move the thumb screw out, eventually it will move past the little springs that cause the "clicking". This is a good sign that you are almost too far. If all your speed ranges seem to be rich, you can also adjust the rack leaner or remove a thin shim under the baro compensator. Either one of these will lean the mixture at all speed ranges. You will probably have to go back and re-adjust the thumb screw inward then to hit the correct mixture on the idle. Your clicking should return once the screw is turned in a bit. Of coarse make sure that your tuning, engine linkages, fuel pressure, and your cold start valve is not leaking before tampering with the IP.

col320ce · « **Reply #11 on:** March 05, 2021, 20:45:51 »

Thanks Joe
Just as you said ... I screwed the idle adjustment in and the clicks started! I have the 2 thick washers and 2 thin... (One looks non original) under the BC making it 4.17 or 4.77mm with/without the non original one. Average pressure here is 1020mb (765mmHg). Without test equipment (that is rare as hens teeth) am I correct in saying that at this pressure the BC should depress the bit it acts on in the fip by 3mm (resting preload) and I should adjust the washer/shims under the BC to keep it at 3mm? (this would equal about 4.7mm of shims)
Then I want to adjust the rack if its still required ... does screwing all the FIP adjustment screws in make them richer and screwing out leaner (ie same direction as the thumbscrew?)
Thanks again

ja17 · « **Reply #12 on:** March 06, 2021, 00:25:20 »

If the BC goes bad a lean situation is usually the result. Since your car seems to be rich, you can probably rule out the BC. Try doing a split linkage test at higher rpms to see if your engine is rich at all speeds. Do it after full warm up.

col320ce · « **Reply #13 on:** March 06, 2021, 00:49:37 »

Thanks again Joe...
I'm starting from scratch and have replaced all the linkage connections. The return spring in the fip is a little sluggish and I think this may be where the richness is coming from. Looks like a PITA to replace!

How is the vertical rod adjusted? I think this is related to gear changing in the autos? I'm pretty sure it does not affect setting idle.

ja17 · « **Reply #14 on:** March 06, 2021, 16:00:18 »

Are you referring to the vertical linkage rod?

Pawel66 · « **Reply #15 on:** March 06, 2021, 18:42:24 »

Quote from: col320ce on March 06, 2021, 00:49:37

Thanks again Joe...
I'm starting from scratch and have replaced all the linkage connections. The return spring in the fip is a little sluggish and I think this may be where the richness is coming from. Looks like a PITA to replace!

How is the vertical rod adjusted? I think this is related to gear changing in the autos? I'm pretty sure it does not affect setting idle.

This spring feels sluggish - that is the way it is. it is not a return spring for linkage, there are two other springs responsible for linkage return.

You may want not to touch this spring at FIP! Nor the shaft it is on. Just adjust the length of the rod.

col320ce · « **Reply #16 on:** March 06, 2021, 23:04:56 »

Yes Joe it's the vertical linkage rod that I haven't worked out how to adjust.

Thanks Pawel for this info... I assumed the fip spring was supposed to be strong enough to act as a return spring. I've put a stronger spring on the venturi and it now snaps back well. All the linkage ends are new now and greased. I'll order the correct spring as I don't want there to be too much tension there as I think things will wear out quicker with too much tension.

I'm hoping it will be a simple matter of tuning it now

Thanks again for helping me work through this

ja17 · « **Reply #17 on:** March 07, 2021, 06:14:54 »

Ok, the socket ends of the linkage rod screw on. One is a right hand thread and one is a left hand thread. There should be an 8MM nut which locks the socket end from moving on it's own. Loosen both ends and twist the rod to change the length.

col320ce · « **Reply #18 on:** March 07, 2021, 06:27:44 »

Is the correct length though in a neutral position? At the moment the vertical linkage is 15mm off neutral position.

Pawel66 · « **Reply #19 on:** March 07, 2021, 11:28:08 »

The FIP arm/lever has to rest against its stop. Then you take the 10mm tube through the hole in the linkage support and if you do not have this hole (you have 230SL), you measure the FIP rod to have 233mm from center of socket to center of socket. Clip this rod to FIP arm ball stud and linkage cross-shaft ball stud. This will now determine the length of the throttle rod - from the other end of linkage cross shaft to throttle ball stud on throttle arm while throttle arm is resting on its stop position.

col320ce · « **Reply #20 on:** March 07, 2021, 12:30:00 »

I understand the the way to set up the fip and venturi linkages. My 2 problems now are that even with the stronger spring the fip does not return fully all the time and I'm tempted to adjust its stop as it's only half a mm. I know it's factory set but I also don't know if it's been fiddled with. This part is causing the variation in idle as it sometimes goes all the way but mostly doesn't. I've tried lengthening the venturi linkage the "force" it home but it doesn't work.
The rod I don't know how to do is the one that runs vertically from the constant speed solenoid to the rhs of the engine. Should this be centered over the balls as well?
Thanks again... I'm definitely getting closer

mrfatboy · « **Reply #21 on:** March 07, 2021, 12:45:28 »

Have you checked the linkage where it attaches to the firewall? There is another spring there.

Pawel66 · « **Reply #22 on:** March 07, 2021, 13:20:38 »

Which of the following is happening:

1. The whole linkage is not going back to stop. When you force it to go back - it does, otherwise the springs are not able to move it back to stop - is that the case?

2. You disconnect the FIP-shaft rod from the shaft ball socket, pull the rod up and still the FIP arm does not come to stop position?

I think these are two very different conditions. I do not know if you can do anything about 2.

If it is 1 - there are number of things to check, starting from checking both return springs (one at the throttle, one at the firewall) and if it does not help - trying to determine what holds the linkage from coming back. There is a number of points to check, including the shaft that goes on the firewall from gas pedal to linkage arm on the passenger side. It could be constant speed solenoid, which for the duration of adjustments should be unbolted and moved aside. All other elements of the linkage such as dash pots, etc., also should be put aside if you have them. It is about determining what is blocking the linkage. Trying to alter anything in FIP shaft - I would not do it.

col320ce · « **Reply #23 on:** March 16, 2021, 13:23:03 »

Update...
The idle is OK at the moment but there are still a few problems.
BTW the fip linkage is now correctly stopping on the stopper.
When warm the idle is mostly at 800 in neutral and 800 in gear... I know this is a little high but the idle fluctuates when driving and sometimes goes as low as 500 and sometimes stalls. What are the common reasons for this?
BTW split linkage test is OK

Also when cold there is a low idle and it wants to stall (and it regularly does stall). I did the split linkage test at startup and it was lean so I added a coke can thickness oval shim under the wrd a now it's a little rich on split linkage test. What are the ways to increase idle when cold... The only fuel adjustment is the shims and there's minimal air adjustment with the round shims. Are there any other cold adjustments that make sense?

Thanks again...

mrfatboy · « **Reply #24 on:** March 16, 2021, 13:46:07 »

You are getting there👍 Have you measured the CO at warm idle? When dialing the mixture in it really helps to know the numbers.

Once you are happy with the warm idle you can adjust the WRD shims to enrich/lean the FIP during warm up like you have. Again, knowing CO numberS really helps. There is a graph of how the CO tracks over time in the tech manual for you to compare to.

The other adjustment is the round shims in the WRD that adjust at what temperature the WRD turns off. Maybe a PO messed with shims and they could be adjusted wrong.

With all that said, automatics have more complexity than manuals with the linkages that can affect idle. Somebody with more experience hopefully will chime in. 😃

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Author Topic: Not another hot start problem! (Read 6542 times)