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post #1 of 115 (permalink) Old 6th March 2006 Thread Starter
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Thumbs up Renault Laguna I - Phase 1 - Heating and Cooling - Possible fix for cold heater

Hi everyone, (and particularly those of you with cold heaters!),

This post relates to a Phase 1 Laguna 1.8 RT (95/96 vintage), conventional heater (i.e. no air-con), but I suspect much of it will apply to other Renault models. While I can see no reason why this approach would be any different on a model with climate control, it would appear that if removal of the matrix ultimately becomes necessary, it is much more difficult / convoluted than on a model bear this in mind.

This job, start to finish took me about 1.5 hours.....but I wasn't rushing.

Usual problem - heater output poor for a few weeks & then nothing, just freezing cold air (which in this weather isn't funny.)

Before I continue I should say that the heater is now belting out bucket-loads of hot air so you might want to read on........(but this is going to be a long-ish post!).

NOTE:- ALSO.....I've included a follow-up at the end of this post which seems to highlight the heater core as a natural 'trap' for a dirty cooling system.......i.e the problem can come back quite quickly. Strong pointer here for giving the ENTIRE system a clean and flush through first otherwise the muck will accumulate in the matrix again.

** Pics added by request, but I'm not up for dismantling it all you'll have to imagine what an exposed heater pipe looks like!

NOTE: The engine needs to be up-to-temp and be sensible - you're dealing with hot water under pressure - don't end up getting scalded!

1) Is the heater matrix actually getting a flow of hot water?

Holding /feeling the heater pipes where they emerge from the bulk-head in the engine bay is probably going to be a red-herring.....they'll get at least warm (& probably hot) from the circulation of the coolant at the top-end in general, (heat conduction).

To really find out, take 5 minutes to pop out the glove box tray - 7 Torx head screws on mine (4 top & 3 bottom - under the bit of 'carpet').

Once out, look through the hole and to the rear / right, you'll see two large-ish (1" dia) pipes angled upwards and disappearing through the bulkhead. Mine are bright aluminium colour.

Get your hand in there and feel both pipes - are they both hot? If not or if one is cool compared to the other then the flow through the matrix is probably restricted or completely blocked, or possibly got an air-lock ( have you been doing anything with the cooling system recently?)

2) Air-Lock:-

This is the simplest / least effort approach thats always worth trying first......(especially if you've been 'doing things' to the cooling system recently).

Don't be too disappointed if it doesn't sort your problem though, simply move to step (3).


Under the bonnet ( just to the left of the brake servo) you'll see the heater connection bayonet....a plastic twin-tube moulding with rubber pipes attached and probably a blue (sometimes black) lever on its right hand side.

On the top hose you'll see a small plastic thumb screw which is the air bleed. With the engine warm and idling (remember....HOT!)....undo this screw ( an old towel is really useful here), and see if coolant OR bubbles emerge. Be careful about taking the screw right out because if the thermostat is open the coolant will almost certainly jet out some 12 or 18".

***Its not impossible for gunge to have blocked the hole, so (with the screw out - CARE!), clear hole with a piece of wire / thin rod.......just to be sure.

If there was lots of air in there, wait until the bubbles stop - i.e uninterrupted fluid comes out, close the bleed screw and re-check the heater.

Is it warm /hot? If so great, top-up the header tank if necessary, refit glove box and you're done.

Heater still not hot? then:-

2) Flushing:-

a) Getting the pipe connection off........

If the engine is now hot, let it cool down a bit and when safe, release the pressure in the system by taking off the header tank cap.

For this next bit, the airbox top and intake air pipe WILL get in your way so take a few minutes to take 'em off and put carefully to one side.

*** cover up engine intake etc with plastic bag or similar to prevent ingress of water during the next bits***

From a number of posts I read here, getting the heater bayonet fitting off presents a problem - it won't move - this is (probably) why:-

The fitting is supposed to pull off if you depress the lever..... no, it doesn't !

The lever itself is NOT the actual catch - but the black plastic bit it is attached to - IS ..........


What you are supposed to do is pull the outer end of the lever away from the connector thereby making the bulkhead end push DOWN on the catch (which is hard to see) ...thereby releasing it

This still didn't work for me - the lever was getting in the way rather than helping. I carefully prised the lever off ( hinges on two small plastic pins).

You will now be able to better see the black plastic lever / catch which actually has a stepped lug at the bulkhead end ( although you can't see that bit). This is what locks the fitting in place and is what is stopping you getting the fitting off.

I found that inserting a flat screwdriver into the outside edge of the opening this tag passes through and levering (carefully) to make the plastic move INWARDS towards the pipe is what releases it. A couple of sharp tugs and the fitting came off exposing the heater matrix inlet and outlet pipes.

Don't lose the 'o' rings IF they pop off.

b) Hose bit:-

Run out the garden hose.

Select whatever nozzle you have that presents a reasonably good fit into the open pipe(s)......(this is also why you need the airbox out of the way!).

Squirt water into the top (bottom) pipe......does water emerge from the bottom and vice-versa?

No? None at all ? Matrix is probably well and truly clogged. Repeat on alternate pipes a few times to see if you can dislodge the obstruction. You are going to need at least some flow at first in order to flush it out.
If you can't get any flow at all - no matter what you do, it might be best to consider a new core at this've already done about 1/3 of the work needed anyway.

Someone suggested elsewhere in this forum to used a compressed air line to blow the debris out.......again your choice, but if the matrix is well and truly blocked, 60 or 70 psi has a good chance of bursting the core.

I did use compressed air once I was sure the core had a reasonable flow through it......and yes it did shift some additional crud.

Mine had a very small flow at first and it was absolutely 'chocker full' with fine red sludge.

c) Treatment:-

The next bit is your choice:-

There are various flushing agents out there, most based on weak organic acids which'll take about a year to de-scale a cruddy heater core - if ever.

I made up an adapter with a cork, a small piece of pipe, a plastic tube and a funnel. This was plugged into the bottom pipe.

Into this I poured about 1 litre of "path clear".....i.e the one that contains dilute hydrochloric acid.....this works on scale very quickly by comparison to "kettle de-scaler".

NEVER, EVER use a liquid that is alkali (caustic) based because alkali materials dissolve aluminium very efficiently.......and the heater core is?......aluminium!

Some material will spill out of the top pipe.....keep the hose handy to keep rinsing off the various bits it WILL splash on. (Goggles are a good idea from here-on in).

Leave for about 5 mins and back flush, using the hose on the top pipe - I bet lots of crud emerges. Reverse the flow.......more crud.....and so on.

I repeated the acid treatment 3 times but on the last time blew the water out of the core first using compressed air......i.e to ensure that ALL the liquid in the core is dilute acid.

3) Finishing off:-

If you are satisfied that you've pretty much got all the debris out that you can, give the core a good run through with clean water to wash out any remaining acid and thoroughly wash down the rear engine bay area.

Jet wash the bayonet connector inlets......they'll be filthy as well.

If you took the (blue?) release lever off, refit it to the bayonet now.

Align bayonet and push fully home on heater pipes - it should click into place.

Dry-off the various engine bay bits and re-fit airbox.

4) Re-filling:-

If you didn't lose too much water, start engine....(top-up a bit if necessary) and run until it gets warm.

Keep opening the heater core bleed valve (care!) to get the air want the thermostat to have opened to really push the air / water through.

When you're happy the air is out, close the bleed screw, top up header tank, (you've lost antifreeze so put some back), check that everything is back in place / tools in their trays.

Get back under glovebox.....are those pipes now both hot?

If so you've almost certainly got your heater back.

Additionally, while you're there, the large black rectangular plastic 'pipe' -just to the left of the heater pipes (see pic no.2), has a circular (3" dia) electrical 'gubbins' set in the top of it?.......this is the blower motor resistor pack. If your blower packs up its almost certainly one of the resistor coils 'gone' or - as is often the case - the thermal fuse inside it. (See other threads on this site.)

Refit glovebox.

Check everything is re-fitted / tight.
Check water levels
Check for stray tools.

Close up car.

Hopefully, you've got heat back again.

Bear in mind that this process might open up leaks in the core if it was best part rotted before you started.

keep a close eye on water levels at first and a 'nose' for nasty damp smells in the car. If you do lose water, check the carpets in the footwells- if wet you'll know need a new core.

Hope you find this useful.......would be interesting to know how many clogged heaters get 'fixed' this way ??



Footnote on Acid Cleaners:-

For those harbouring concerns about dilute acids......

Most de-scalers etc, are based on weak organic acids such as Acetic acid(vinegar if you like) or citric acid('lemon juice'). These materials will dissolve carbonates such as lime-scale, but they are chemically classified as 'weak' acids and as such the process is slow. They do not dissolve rust deposits but might help loosen them. Because of the 'gentle' action they are deemed safe for the general public and are unlikely in the extreme to harm anything they are put in.

Mineral acids, (Hydrochloric, Nitric, Sulphuric etc), are deemed strong acids and in their concentrated form are aggressive and hazardous. In the case of 'path clear' type materials, Hydrochloric acid is the active agent, and as supplied is already diluted to about 5 or 10% with water. At this strength it will vigorously dissolve lime-scale/carbonates and will attack many metals including aluminium. I tried the solution (above) on a piece of freshly abraded Ali' first - the reaction was very very slow, however it rapidly dissolved lime-scale - which is what we are after.

It does not readily dissolve copper although the brown /black tarnish (oxide) may be removed.

It will also irritate the skin (itch) so wear protective clothing / goggles and have a hose to hand to quickly wash off any spills/splashes.

In the case of a blocked heater (which nowadays is likely to be made of Aluminium), any contact with dilute Hydrochloric acid should therefore be kept to a minimum, (say 5 to 10 mins total). I decided that the approach with ours was 'kill or cure' was useless anyway!

If in doubt try to blow most of the water out of matrix with compressed air (providing its not completely blocked - see above), and back-fill with 'kettle de-scaler' or 'Rad-Flush' and leave for an hour or so......then back-flush with the hose. It may(?) be enough with yours.

Alkali:- Caustic soda / Sodium Hydroxide / oven cleaner/ 'Lye' ........ vigorously attacks Aluminium - do NOT go there!


One week into 'repair'.......Heater working fine and no leaks!


Ten days into 'repair' and heater getting colder again !!! Took off connector (getting a dab hand at this - 8mins), and lo and behold MORE sludge. The heater must act as a trap on this system OR more debris has loosened-up. Quick backflush with hose and all fine time 22 minutes.


11 days in and would you believe it.....heater getting cold again AND more brown sludge. This is clearly debris from the rest of the system 'collecting' rapidly in the heater core which is a low point on the system.

Put a large bottle of 'Rad Cleaner' in the system and took it for a 20 mile drive. Disconnected top hose at stat' end (easy) and pumped block through with a pressure hose jammed in the rad's top hose.....(stat will close when cold water reaches it though). Disconnected bottom rad hose at engine block end (again, easy) and pumped water through the rad.......a lot more rust sludge came out. Continued until water ran clean then re-connected and re-filled. Backflushed heater (again) for good measure.


14 days in and heater now hotter than it has been in a LONG while - in fact on 'full' it becomes stifling and uncomfortable. Thats the ticket!


4 weeks 'in' and still fine!


6 months in and still A-OK
Currently in spirochete's garage:
Jaguar V8 Sport / Mercedes 'E" / Laguna 1.8RT

Last edited by spirochete; 19th January 2007 at 10:43 PM. Reason: more updates
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post #2 of 115 (permalink) Old 6th March 2006
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Excellent post to what is a very common problem.

Pictures would be welcomed ?
post #3 of 115 (permalink) Old 6th March 2006
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Post of the year...So far.. In valuable advise..

Many many thanks for reading my PM'd instructions on how to post pictures -

Maybe I should make a seperate thread for ppl who do not know hot to include pictures in their posts ?
post #4 of 115 (permalink) Old 6th March 2006 Thread Starter
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Not a bad idea.

Caught me out for a while.......but easy when you know how.

As they picture is worth a thousand words..........

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post #5 of 115 (permalink) Old 6th March 2006
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Originally Posted by Laguna II Owner
Excellent post to what is a very common problem.

Pictures would be welcomed ?
hi there some great adive there,i wonder if u can help me? i have a 2.0 laguna with air con ive done most of the things u said apart from the acid flush only had the car a few months so i dont want to take the chance of an acid flush anywat ive tried to replace the matrix,the way through the glove box and ive had the dash out with no luck i cant get the matrix passed a big black box air distrebution box i think any ideas cheers phil
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cold , cooling , fix , heater , heating , laguna , phase , renault

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