Hard warm start

[zedster]

Anyone know what would cause a hard warm start? I recently had my fuel distributor replaced and WUR is next. When the car is warm it will take 2 or 3 turns of the key to get her to start. It was not like this before the fuel distributor was replaced.

[koan]

A leaking fuel distributor metering piston or leaking injectors.

koan

[zedster]

Koan would you lean towards the leaking injectors since the fuel dist has just been replaced?

[koan]

Koan would you lean towards the leaking injectors since the fuel dist has just been replaced?

There are rich hard starts and lean hard starts. Rich is when fuel leaks into the manifold when not running, lean is when fuel pressure drops when not running.

Could be that the rebuilt fuel distributor is a leaking. Seems too much of a coincidence that injectors would begin to leak at same time as fuel dist replaced.

The fuel accumulator maintains fuel pressure while the engine is off and is often blamed for warm hard starts but affects cold starts as well. A check is to put the auto in something other than "P" or "N" and turn the key to start for 5 seconds, the engine won't crank but the fuel pump will run. If this fixes the hard start, it's the fuel accumulator.

koan

[s class]

It may be hijacking the thread, but my 450SL k-jet has hot start problems too - so maybe a discussion of it may also assist the original poster. 

My symptoms (not sure if exactly the same as zedster), are :

On cold start :

starts quickly and easily, and idles well for the first few seconds, then stumbles a lot and wants to cut out, until its warmed up a bit.  I explain this as being due to the cold start valve helping initially.  Once the cold start valve has stopped injecting, the stumbling starts, and I have found that during this period, the control pressure is too low.  Once it warms up, the control pressure increases, and the idle improves.  At operating temp, the control pressure is still a bit low, but not as badly so as at cold start.  During the cold phase with stumbling idle, the exhaust smells very rich, which confirms that the control pressure is too low (as I understand things).

On hot start :

Takes a lot of cranking, but when it does fire, it idles very well.   I have also found that once the car is hot, and turned off, even after only 10 mintues (ie not much drop in engine temp), the control pressure has dropped a lot lower than I would expect - almost back to the cold start values.  I have tried the priming the fuel pressure with gear shift in "D" position trick, and it made no difference to the difficult hot start, so I don't think the accumulator is the problem in my case. 

On normal running at operating temp : No obvious problems.

Given that the control pressures appear wrong when cold, I favour the WUR as prime suspect.  Under hot conditions, the way the control pressure drops rapidly after turning off the engine appears incorrect, though I'm not certain it is incorrect - opinions?  If it is incorrect, it again suggests the WUR may be to blame. 

Any other opinions on the likelihood of the WUR being the culprit, or should I be looking at the FD?  To be fair, i haven't yet checked some of the basics like the inlet screens to the FD and WUR. 

The car has been like this since I got it 4 years ago, the symptoms have got neither better or worse.

Thanks

[koan]

On cold start 

Does your engine have an idle up valve?

A valve that off the back off the back of the K-Jet air metering housing, two large hoses and one small vacuum line. Adds extra air around throttle plate straight after starting for a few seconds. In warm climates does the cold start valve ever inject, easily tested by disconnecting it.

Could it be just idle RPM too low due to stuck aux air valve?

On hot start :

If The "D" pre crank doesn't help, the difficult start is most likely due to build up of fuel in the manifold from fuel distributor or injector leak.

Could also be cold start valve leaking constantly which I forgot to mention in previous reply.

Fuel pressure should drop when engine is shut off to just below injector opening pressure but shouldn't continue to drop towards zero. Control pressure can't be higher than fuel pressure.

Is control pressure correct at normal operating temp?

if not it can be increased by tapping down the pin. When correct at operating temp what you get cold is what you get and can't be adjusted. Decreasing control pressure requires opening up the WUR and pushing up the pin.

Blocked WUR inlet screen results in high control pressure and lean running.

My car has developed the warm start problem, under all other conditions it's fine. I have a spare fuel distributor that I should have tried but in the 'interest of science" I opened it up and now needs new o-rings and resealing.

koan

[s class]

koan, thanks for an insightful reply.  I appreciate it. 

Does your engine have an idle up valve?

A valve that off the back off the back of the K-Jet air metering housing, two large hoses and one small vacuum line. Adds extra air around throttle plate straight after starting for a few seconds.

To be honest, I'm not sure.  Its dark now (10pm) so I'll have a look tomorrow morning. 

In warm climates does the cold start valve ever inject, easily tested by disconnecting it.

I'm not sure.  But you are correct in asking - its a relatively simple check I can make.  I did once a few years back check, and confirmed that the CSV does not leak or drip once the car is operational. 

Could it be just idle RPM too low due to stuck aux air valve?

I don't think this is a problem.  I did remove the AAV and boil it on the stove.  I did see the full range of movement from completely open to completely closed. 

On hot start :

If The "D" pre crank doesn't help, the difficult start is most likely due to build up of fuel in the manifold from fuel distributor or injector leak.

I fear that you may be correct in this case.  I confess that I simply haven't put in the time required to test the injectors etc. 

Could also be cold start valve leaking constantly which I forgot to mention in previous reply.

No, see above, I'm pretty certain this is not a problem. 

Fuel pressure should drop when engine is shut off to just below injector opening pressure but shouldn't continue to drop towards zero. Control pressure can't be higher than fuel pressure.

That makes sense.  I did once do a check where I monitored the system pressure, and watched the pressure decay rate after shutting off the engine.  the decay rate was very slow, and the residual pressure was within spec.... so on second thoughts that probably does suggest its not a leaking FD or injector issue....

Is control pressure correct at normal operating temp?

No - its about 0.5 bar low - but then so is the system pressure a little low.  As I understand it, I should correct the system pressure first, and then re-check control pressure before messing with the WUR. 

Blocked WUR inlet screen results in high control pressure and lean running.

I hadn't really thought about that, but what you say makes sense.  it suggests blocked screens are not my problem, but it probably wouldn't hurt to clean them to ensure basic problems are eliminated. 

My car has developed the warm start problem, under all other conditions it's fine. I have a spare fuel distributor that I should have tried but in the 'interest of science" I opened it up and now needs new o-rings and resealing.

Well I did eventually get pompy's solidly gummed up 450SE FD apart after soaking in solvent, and I sent in the cast parts to be ultrasonically cleaned.  they showed me the residue that came out, and the quantity of muck was something to be marvelled at.  I will try to get new seals for it either Thursday or Friday, so that i can reassemble it (or at least attempt) on Saturday.  I think I have convinced myself to attempt this without any sealant between the halves of the FD.  I will be relying on perfectly clean surfaces, just as when they were first assembled. 

[koan]

I think I have convinced myself to attempt this without any sealant between the halves of the FD.  I will be relying on perfectly clean surfaces, just as when they were first assembled. 

Not sure about no sealant being used. The one I pulled a part had a very thin and very even coat of something on the four surfaces, looked almost like a discolouration but it came off with carb cleaner.

Saw somewhere to use "Permtex Indian Head Shellac", I found a bottle of it on the shelf at Burson's. It is quite thick, I intend diluting it with a suitable thinner and using a foam paint roller to put an even coat on the two halves but not the shim.

That's when I find time that is.

koan

[s class]

Ah, that's an interesting way of getting it even.  Using some sort of sealant would at least give one a fighting chance of getting it to seal again.  Perhaps I need to revisit what sealant options might be available to me locally. 

[johnnyw116]

Ah, that's an interesting way of getting it even.  Using some sort of sealant would at least give one a fighting chance of getting it to seal again.  Perhaps I need to revisit what sealant options might be available to me locally. 

hi i know its off topic but i noticed 107-044 euro 450SL i have an W107-044 euro SL too but mine is an 280 SL ? 

greetings : johnny  .               

[s class]

107.044 should be a 450SL, unless its had an engine change. 
107.042 is a 280SL

[zedster]

WUR has been replaced and has helped a lot. The hard warm start is still there and I noticed today that while turning they key and hitting the gas pedal a bit it turn over a lot quicker. My mechanic is going to check out the injectors later this week.

[s class]

I have FINALLY located a local supplier for Permatex "High Tack Spray-A-Gasket Sealant", which is claimed to be highly resistant to fuels and can handle a wide temp range.  I intend to use this on pompy's FD. 

[Nutz]

I think the accumulator is at fault.

[s class]

Nutz, are you referring to the small accumulator next to the FD, or the large unit at the rear by the pump?