CX C-Matic shuddering & stalling in reverse?
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Thread: CX C-Matic shuddering & stalling in reverse?

  1. #1
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    Default CX C-Matic shuddering & stalling in reverse?

    Hey guys,

    My CX2400 C-matic (1979, carburettor) has recently been shuddering and stalling out in reverse, especially when not fully warmed up. It's fine in any forward gear and runs smoothly, but if I put it into reverse without the choke fast idle engaged, it gets really shuddery and usually stalls after I move a metre or two.

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    My transmission fluid level is correct, although I'm fairly certain the previous owner was adding the wrong fluid - a nulon semi-synthetic multi-vehicle fluid (https://www.nulon.com.au/products/au...smission-fluid). Should I replace all the fluid with Transmax type F?

    The idle is consistently about 800-1000 rpm (choke off) for any forward gear but is slower for reverse, even with choke on.

    hope you can help, thanks very much.
    max.
    79 cx2400 pallas c-matic

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    1000+ Posts gerrypro's Avatar
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    Quote Originally Posted by 37dhw8 View Post
    My transmission fluid level is correct, although I'm fairly certain the previous owner was adding the wrong fluid - a nulon semi-synthetic multi-vehicle fluid (https://www.nulon.com.au/products/au...smission-fluid). Should I replace all the fluid with Transmax type F?
    Yes change out the fluid to Transmax F. It will need several flushes to remove the fluid from the torque converter. Check the usual other culprits ----- points clearance for the gearchange switch box( on to of the gearbox near the selector mechanism, change the trans oil filter after the last flush out, increase the idle RPM under step up conditions and make sure all the hoses in the vacuum circuits are in good condition and the there are no vacuum leaks.
    Cheers Gerry

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    It does need the choke when cold. In the CX it's an entirely manual choke unlike some more complex carby setups.

    I once had an issue where the wire from the coil to the distributor would sometimes short out on the block when reverse was engaged and stop the car dead when the engine moved enough on the mounts. It would then restart as normal. The insulation had rubbed through and it wasn't obvious. It may not be your issue, but it takes only a moment to check.
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    1000+ Posts gerrypro's Avatar
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    Another problem may be a low voltage issue causing the idle cut off solenoid on the back of the carb to close stalling the motor. To cure this one a lot of people remove the valve from inside the solenoid. Sometimes all it takes is an RPM increase as stated earlier.
    Cheers Gerry

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    Gerry - thanks to your detailed input, I think I've found the issue - the idle step up valve isn't actuating. The little vacuum switch thing IMG_0873 s.JPG (this) is blocked off at one end - what is it supposed to attach to ?

    David - funnily enough I used to have that issue a while ago, but have since rectified it. Thanks for the help though.

    Thanks Max.
    79 cx2400 pallas c-matic

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    Quote Originally Posted by 37dhw8 View Post
    Gerry - thanks to your detailed input, I think I've found the issue - the idle step up valve isn't actuating. The little vacuum switch thing IMG_0873 s.JPG (this) is blocked off at one end - what is it supposed to attach to ?
    Thanks Max.
    The outlet on centre feeds vacuum to the clear plastic capsule on the ventilation tower and then to the step-up valve on the carb.
    That should be the one that is not blanked off in your pic.
    The outer feed that is blanked off should pick up a vacuum source from the carb base. There are a few T pieces but basically it also draws from the EGR plumbing of the crank case.
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    Cheers Gerry

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    Thanks Gerry, that turned out to be the issue. All of the hoses were in a tangle, although I still am not totally clear on what the vacuum system should look like - there is one other vacuum control thingy, like the one in the picture, which was routed through the plastic capsule into the backside of the carb. What should plug into the other port on the switch ?

    Thanks Max.
    79 cx2400 pallas c-matic

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    1000+ Posts gerrypro's Avatar
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    Sorry Max, My mistake! That one is associated with the antipollution gear. It feeds to a Diennes valve low down on the left chassis rail and also to a vacuum solenoid valve on the exhaust air supplement system. Many CX's have long dispensed with this anti pollution gear. If your car still has the belt driven air pump then this vacuum solenoid would still be active. If the airpump has been removed then it would be blanked off and no longer necessary.
    Cheers Gerry

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    Ah no problem ! You've forgotten more about CXs than I will likely ever know...

    The anti pollution stuff is long gone, but is that related to the vacuum line going into the rear of the carb?

    Thanks, Max.
    79 cx2400 pallas c-matic

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    Quote Originally Posted by 37dhw8 View Post
    Gerry - thanks to your detailed input, I think I've found the issue - the idle step up valve isn't actuating. The little vacuum switch thing IMG_0873 s.JPG (this) is blocked off at one end - what is it supposed to attach to ?

    David - funnily enough I used to have that issue a while ago, but have since rectified it. Thanks for the help though.

    Thanks Max.
    I'm a bit puzzled by your diagnosis. I've never had the issue you describe, and only in reverse is odd to me. What happens if you start in 2nd gear instead of first?

    The idle step up valve is actuated by AC and this works on my car. I found something in a manual years ago about a step up actuation when a gear is engaged with the C-matic, but I haven't ever noticed this working on my car. When the AC idle step up stopped working on my car, it didn't affect reverse gear.

    Let's hope your solution works of course. If so, and if there's an idle step-up with the C-matic, does your work?

    I sort of expected the issue to be distributor points/timing and carbie settings.

    Does it happen when the car is fully warmed up?

    So, please post more when it is fixed as some of us as usual will have something new to learn!

    Cheers
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    JohnW

    Renault 4CV 1951
    Renault R8 1965
    Renault Scenic 2005 (wife's)
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    Renault Scenic 2006 (daughter's)
    CitroŽn CX Pallas 1980

    National Co-ordinator, Renault 4CV Register of Australia

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    Quote Originally Posted by JohnW View Post
    I'm a bit puzzled by your diagnosis. I've never had the issue you describe, and only in reverse is odd to me. What happens if you start in 2nd gear instead of first?

    The idle step up valve is actuated by AC and this works on my car. I found something in a manual years ago about a step up actuation when a gear is engaged with the C-matic, but I haven't ever noticed this working on my car. When the AC idle step up stopped working on my car, it didn't affect reverse gear.

    Let's hope your solution works of course. If so, and if there's an idle step-up with the C-matic, does your work?

    I sort of expected the issue to be distributor points/timing and carbie settings.

    Does it happen when the car is fully warmed up?

    So, please post more when it is fixed as some of us as usual will have something new to learn!

    Cheers
    Hi John ! Honestly, at this point, as long as it goes i'm not going to do anything. "If it ain't broke, don't fix it" is my mantra at the moment (at least that's how I'm justifying my relative lack of savoir-faire...)

    I seem to have found a vacuum control capsule that activates a step-up valve when the car is put into gear. I've connected it as Gerry described, and it seems to be working well now. The aircon doesn't work in my car anyway, so I suppose I should investigate this properly when I have sorted that out.

    I think really it happens to some degree in all gears, but it's just very very apparent in reverse. Doesn't happen when the car is totally warmed up, but is still slightly noticeable in reverse. With the step up valve, however, I am able to keep the idle speed consistent when in gear which practically eliminates the original issue (idle dropping too low when in gear at cold - instead the throttle is just held at a consistent position).

    I have always suspected the car wasn't set up quite right ... The carb was VERY strangely tuned, with a very very rich fuel mix but the idle speed screw was totally backed out. Fixed that up and tuned up by ear, great improvement. Recently replaced and adjusted all ignition components including timing, so I don't think that's a problem, but it sort of was when I got the car.

    An absolute puzzle (as usual...)

    Regards Max.
    79 cx2400 pallas c-matic

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    Always hard when you can't know what's been done by previous owners! I agree your first sentence! There are usually higher priority things to fix....

    My understanding is that the vacuum capsule you've found is actuated by both AC and the C-matic, but mine has never been actuated by the C-matic. Since it behaves without that refinement, I've never bothered to find and fix the sender - I suppose it is actuated by the switch block on the gearbox.

    If you PM me your email address, I'll send the Mug's Guide to the handbrake adjustment that I prepared a couple of weeks ago.

    I had some professional input to the AC last year, and it is going well with a new condenser and new compressor. The previous owner did a good job of fitting a larger evaporator into the cabin unit. A modification I have done is to get two new double belt pulleys machined up so I have a better drive to the compressor, and without ludicrously tight belts needed to prevent slippage.

    Cheers
    JohnW

    Renault 4CV 1951
    Renault R8 1965
    Renault Scenic 2005 (wife's)
    Renault Scenic 2007 (mine)
    Renault Scenic 2006 (daughter's)
    CitroŽn CX Pallas 1980

    National Co-ordinator, Renault 4CV Register of Australia

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    Quote Originally Posted by JohnW View Post
    Always hard when you can't know what's been done by previous owners! I agree your first sentence! There are usually higher priority things to fix....

    My understanding is that the vacuum capsule you've found is actuated by both AC and the C-matic, but mine has never been actuated by the C-matic. Since it behaves without that refinement, I've never bothered to find and fix the sender - I suppose it is actuated by the switch block on the gearbox.

    If you PM me your email address, I'll send the Mug's Guide to the handbrake adjustment that I prepared a couple of weeks ago.

    I had some professional input to the AC last year, and it is going well with a new condenser and new compressor. The previous owner did a good job of fitting a larger evaporator into the cabin unit. A modification I have done is to get two new double belt pulleys machined up so I have a better drive to the compressor, and without ludicrously tight belts needed to prevent slippage.

    Cheers
    Glad we are on the same page ! So long as it works... I have the registration appointment later today so let's hope it doesn't go embarrassingly wrong...

    The previous owner supposedly fitted all now components to the AC system, and to my knowledge everything works well, but the only problem is the switch inside the cabin that's supposed to switch on the compressor clutch. There are loads of wires leading out of it, but i can't find the corresponding one in the engine bay to go into the relay for the compressor clutch ! Time for some diagnostic action ... putting it off til next summer ...

    I'll send you a PM. Thanks !

    Max.
    79 cx2400 pallas c-matic

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