Jumping forward and poor changeover CLS500

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reddwarf
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Jumping forward and poor changeover CLS500

#1 Post by reddwarf » Tue Dec 31, 2019 3:36 pm

Hello,

I recently had my Mercedes CLS500 (M113 engine) converted to LPG, but I'm suffering problems when the car changes over to LPG and when coming to a stop. The system has an ALEX ECU and Barracuda injectors. When changing from petrol to LPG (which occurs at 1200 RPM) the car judders, it feels almost like a misfire. I've got a diagnostic cable and software for the LPG system and even when changing one injector at a time from petrol to LPG the judder still occurs. I've replaced the spark plugs, but haven't touched the HT leads or coil packs.

The main issue I'm suffering is when coming to a stop the car will jump forward and the revs will bounce. I initially thought this could be a vacuum leak, but I've since had this looked at and there are no leaks to be seen.

Any thoughts? Thanks

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Re: Jumping forward and poor changeover CLS500

#2 Post by Brian_H » Tue Dec 31, 2019 4:03 pm

Whereabouts have the injectors been mounted and how far are the pipes that come from them to the inlet manifold, and where do they connect to the manifold?

Thinking that the pipework could be longer than it should be, or the spud positioning (the spuds being the bit where the gas enters the manifold) could be too far from the petrol injectors, causing a lag effectively (as it has to fill the pipework with gas before it gets to the manifold).

Photos may be easier to provide an answer than a description, if you click the full editor & preview button from the reply window, you can add photos directly on the forum.

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Re: Jumping forward and poor changeover CLS500

#3 Post by reddwarf » Tue Dec 31, 2019 4:40 pm

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Re: Jumping forward and poor changeover CLS500

#4 Post by LPGC » Tue Dec 31, 2019 4:47 pm

Lots of possibles, some already mentioned by Bri, another one worth mentioning is that the installer may have set the car to not only wait until 1200 rpm before it changes to LPG but to also switch back to petrol at anything below (say) 1200rpm or switch back to petrol on over-run and then have to switch back to LPG for idle again. Such settings could be done in a way that gives the driver the impression (LPG switch lights) that it continually runs on LPG after the 1200rpm switchover even though really it's switching between fuels around that rpm threshold. If you get an unsmooth changeover from petrol to LPG you might also get an unsmooth changeover from LPG back to petrol (and if it switches back to petrol momentarily during over-run you'd have the unsmooth changeover every time it switches back to LPG anyway).

Edit - hehe just seen your pic... yes you have long pipe lengths there! Like Bri says the long pipe lengths won't help at all with smooth running at low rpm or with smooth changeover, but sometimes installers will set the system to switch back to petrol when they can't get them to idle nicely on LPG (due to long pipe lengths etc). You could have both a long pipe length problem and an ECU settings problem in which case solving just one problem won't make for a good conversion.

What year is the car? Early(er) versions of the petrol system on these engines (as in S500's etc) have a conventional mechanical controlled petrol return system, later versions (most CLs) are returnless and on returnless engines petrol pressure needs to be addressed (best way by fitting a physical fuel return) for good results on LPG to be possible. No sign of a fuel return being added in your engine bay (would usually be fitted into the petrol pipe between engine bay and engine), electronic 'fixes' are available but are not as good as a proper physical fuel return.. that's if your engine needs a fuel return.

I'd expect to find more than one problem tbh, even if shorter pipes seems to fix everything I'd still expect the installer to have set it to switch back to petrol at idle or after over-run (so would use more petrol than you ought to be using). Seems the installer fitted the injectors with long pipe runs because they're too big to fit in a much better place that would keep pipe runs shorter... But they could have fitted smaller injectors and could have removed the hard plastic case around the engine loom to fit flexible protection around the loom instead (allowing smaller injectors to fit in a better place to keep pipe lengths shorter). The reason they didn't will be because they limit themselves to using just a few types of chunky injectors and the simple wiring loom mod didn't occur to them.
Last edited by LPGC on Tue Dec 31, 2019 5:08 pm, edited 1 time in total.
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Re: Jumping forward and poor changeover CLS500

#5 Post by reddwarf » Tue Dec 31, 2019 5:04 pm

The car is a 2005. To my knowledge they only fitted the M113 to the earliest of the CLS models before changing to the M273 engine in 2006/7, so I think this is one of the later M113's Merc made.

I've had a look in the software and I would assume that if it was set to return to petrol at idle it would be on this screen?
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Re: Jumping forward and poor changeover CLS500

#6 Post by LPGC » Tue Dec 31, 2019 5:11 pm

They haven't set the ECU to switch back to petrol at low rpm then... but it could still be doing so if the LPG ECU loses the positive 'ignition on' signal on over-run, this would depend on where they'd connected that (ignition on) wire to.

If your code reader shows live data (fuel trims etc) can you make it show fuel pressure? If you can then it needs fuel pressure addressing.
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Re: Jumping forward and poor changeover CLS500

#7 Post by reddwarf » Tue Dec 31, 2019 5:50 pm

I've had a look at my code reader and unfortunately it can't show fuel pressure. If I used a pressure gauge and attached it to the Schrader valve on the fuel rail what should I be looking for in order to diagnose this? Also, trying to fix this, e.g. moving the injectors, shortening the pipes, possibly adding a fuel return and then correcting the LPG map, is probably going to be out of what I'd be comfortable doing myself. Could you recommend a competent installer in and around Somerset?

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Re: Jumping forward and poor changeover CLS500

#8 Post by Brian_H » Tue Dec 31, 2019 11:32 pm

reddwarf wrote:
Tue Dec 31, 2019 5:50 pm
I've had a look at my code reader and unfortunately it can't show fuel pressure. If I used a pressure gauge and attached it to the Schrader valve on the fuel rail what should I be looking for in order to diagnose this? Also, trying to fix this, e.g. moving the injectors, shortening the pipes, possibly adding a fuel return and then correcting the LPG map, is probably going to be out of what I'd be comfortable doing myself. Could you recommend a competent installer in and around Somerset?
What you'd be looking for is to compare the pressure reading on gas and petrol - if its rising and higher on gas, then you may need the fuel return (as the petrol ecu will adjust petrol injection timing to compensate for the pressure in the fuel rail - shorter injection for higher pressure giving the same amount of fuel, doesn't take much to realise thats fine when its running on petrol, but then trying to convert that to something sensible for gas when the gas pressure isn't referenced to the petrol pressure gives you a bit of a problem, the fuel return effectively allows the pressure to stabilise when its on gas to give you a consistant base to work out the gas timings).

What are you using as a code reader? dedicated unit, pc software or a phone app? If its a phone app, then Torque might work and that can display fuel pressure (don't know if your car will speak to it though, or if it will report the fuel pressure). Do you have a field for the pressure visible but no data displayed, or does it not have a field?

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Re: Jumping forward and poor changeover CLS500

#9 Post by reddwarf » Tue Dec 31, 2019 11:59 pm

I've tried using both Carly and Torque Pro but the only related field I can read is fuel rate (cc/min). I'll try and have a look tomorrow as to see whether I can diagnose the potential fuel pressure issue.

The car generally runs fine under load it's just during change over and overrun that these issues occur. Would the lack of a fuel return pose problems at all times during running? Is the issues I'm describing likely to be a combination of long pipe length and the lack of a fuel return?

Thanks

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Re: Jumping forward and poor changeover CLS500

#10 Post by Brian_H » Wed Jan 01, 2020 8:02 pm

might be an easier way to identify if there is a fuel return actually, look under the car and find how many pipes run to the tank (they will be the thicker ones your looking for). If theres 1, its unlikely you have a fuel return, if theres 2 then you probabbly do.

It may be that you don't have a fuel pressure sensor, which would be why you can't find the value for it. I can find it in torque pro for both fuel pressure and fuel rail pressure as shown in the screenshot, so if you can't find them, or if they aren't green it may be that your car doesn't have sensors for them.

Potentially yes, both will cause issues, the pipe lengths definitely look far too long to me (30cm is the general recommended maximum, while the shorter you can get them the better really) It also looks to me like some of them are much longer than others, which also isn't ideal.
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Re: Jumping forward and poor changeover CLS500

#11 Post by LPGC » Thu Jan 02, 2020 10:17 am

Don't forget a second pipe running to the tank could be the evap purge pipe, a lot of cars have 3 pipes running to the tank... feed, return and evap purge.

Don't concentrate on a possible fuel pressure issue for now, instead concentrate efforts on relocating injectors to make pipe lengths shorter. If there's a fuel pressure issue it will be evident later on and can be addressed later on - If you calibrate the system so fuel trims are the same on both LPG and petrol at idle then if there isn't a fuel pressure issue pinj will remain the same, or if there is a fuel pressure issue pinj will change quite a lot for same fuel trims. You need to be able to idle smoothly on LPG to make the worthwhile pinj / trim comparison so sort the pipe lengths first.
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Re: Jumping forward and poor changeover CLS500

#12 Post by reddwarf » Thu Jan 02, 2020 10:58 am

Thanks for the help everyone.

I've contacted an installer that I've heard good things about and hopefully they should be able to get the injector pipe issue sorted. I'll update hopefully once I've had the work done.

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Re: Jumping forward and poor changeover CLS500

#13 Post by reddwarf » Fri Jan 17, 2020 10:58 pm

LPGC wrote:
Thu Jan 02, 2020 10:17 am
Don't forget a second pipe running to the tank could be the evap purge pipe, a lot of cars have 3 pipes running to the tank... feed, return and evap purge.

Don't concentrate on a possible fuel pressure issue for now, instead concentrate efforts on relocating injectors to make pipe lengths shorter. If there's a fuel pressure issue it will be evident later on and can be addressed later on - If you calibrate the system so fuel trims are the same on both LPG and petrol at idle then if there isn't a fuel pressure issue pinj will remain the same, or if there is a fuel pressure issue pinj will change quite a lot for same fuel trims. You need to be able to idle smoothly on LPG to make the worthwhile pinj / trim comparison so sort the pipe lengths first.
Hello again,

I've since had the LPG injector pipework shortened and although the car is driving a lot better (much more responsive on gas), I'm still having issues at overrun, where the car will judder or hesitate after coming to a stop. I've since checked and I'm fairly certain that the car does not have a physical fuel return. This being the case what is the next steps I have in diagnosing whether fuel pressure is potentially causing an issue?

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Re: Jumping forward and poor changeover CLS500

#14 Post by LPGC » Sun Jan 19, 2020 10:01 pm

Monitor the 'fuel rail pressure' and 'fuel rail relative to manifold pressure' readings during over-run, to see if the readings climb or fall dramatically. If they fall very low it will be because the car didn't originally feature a fuel return but the LPG installer fitted a physical return which is still active during over-run and the fuel pump turns off during over-run. This would have the effect of over-richening the mixture when running on LPG following over-run because of the petrol system compensating for low fuel pressure by increasing pinj. There would be various potential fixes including: Using an LPG ECU with dedicated programmable switched output to control a petrol fuel return solenoid (disable solenoid when pinj is zero), or using an LPG ECU that features a 'switch back to petrol after long cut off' type feature, or making some custom electronics that disable a fuel return solenoid when petrol injectors are not pulsed, or fitting an electronic fuel pressure emulator (still the best idea to also have a physical fuel return fitted).

If the above isn't the problem the problem may just be a simple calibration issue but could also involve an LPG setup/design issue (such as injector nozzle / pressure combination that calls on injectors to pulse for too short a duration).

Edit - despite the points I've mentioned I'd regard conversion of this vehicle as being rather straight-forward, the problems you're having don't bode well for this installer especially if they don't know how to address the simple points properly.
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Re: Jumping forward and poor changeover CLS500

#15 Post by reddwarf » Mon Jan 27, 2020 3:06 pm

I've slowly been working through solving some other problems with the car and now the LPG system has my full attention. I've noticed the installer has connected the vaporizer and the LPG MAP sensor to a vacuum line that feeds the secondary air injection system. In doing so he has removed a one-way-valve that is meant to be fitted, that I obviously want to replace.

My question is, where do these components have to have their vacuum lines connected to? Should it be taken from the same vacuum line that feeds the petrol MAP sensor or from a separate port in the manifold. My manifold has a spare vacuum port that could be used although is on the back of the manifold. What is the maximal permissible distance for the vacuum hose that feeds both the vaporizer and LPG MAP sensor, as if I use the spare vacuum port the length of the hose might be around 50cm for it to reach both LPG components?

Thanks again

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Re: Jumping forward and poor changeover CLS500

#16 Post by Brian_H » Mon Jan 27, 2020 6:46 pm

I wouldn't be concerned at that distance with the vacuum line, thats reasonably short compared to some installs. I'd go for a seperate port personally as then you know its only going to those bits of kit and not causing any other unknown possible problems like what you've discovered with the missing oneway valve. You can usually get the map sensor fairly close to the manifold anyway even if the vapouriser isn't particaully close.

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Re: Jumping forward and poor changeover CLS500

#17 Post by LPGC » Mon Jan 27, 2020 9:58 pm

Ye standard fairly old-skool Merc V engine manifold - on these motors I usually T into the vac pipe that connects to (in your engine pic) the vac reservoir at front left of the engine.
It's difficult to access the rear end of the manifold's plenum close to the throttle body (which would be the usual place to fit the vac pipe) and it's a good idea not to drill through other parts of the manifold, some of which contain variable inlet tract length gubbins that you don't want to prevent moving by installing a vac nozzle through both the (outer) manifold and (inner) variable tract gubbins.
Find the vac pipe that connects to the manifold vacuum (not any of the others that are solenoid controlled) and T into that... The fitting for this vac pipe connects to the plenum of the manifold rather than any of the port runners, so it isn't much of a detriment (it's still into the plenum) to use this vac source instead of a custom vac source close to the throttle body.
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