Disaster! Very ill Bongo - 2 litre

[francophile1947]

Simon, surely the valves won't open and close with a broken cambelt, as the camshaft will not be turning If the valves aren't opening and closing, some cylinders will never have any compression until the camshaft turns to close the valves.

[rita]

Using the starter motor .

http://www.popularmechanics.com/cars/ho ... t-14912158

A compression test reveals the condition of your engine's valves, its valve seats, and piston rings and whether these parts are wearing evenly. Healthy engines should have compression over 100 psi per cylinder, with no more than 10 percent variation between the highest and lowest readings. With a compression tester, a few hand tools, and 20 minutes, you can try this yourself.

STEP 1 Remove the fuel pump and fuel-injection fuses. Disconnect the main wire to the coil and spark plug wires; remove spark plugs.

STEP 2 Start the threaded end of the compression gauge in a spark plug hole by hand.

STEP 3 Turn the ignition on, depress the throttle, and crank the engine four revolutions. This should result in a stable reading; if not, crank up to 10 revolutions, but do the same with all cylinders.


STEP 4 Mark the pressure reading for each cylinder on the valve cover in chalk, then move to the next cylinder.

TIP For a cylinder below 100 psi, pour 1 teaspoon of engine oil into the plug hole and retest. If the reading jumps, the piston rings are worn. If not, think valve problems.

You cannot test the compressions with a broken (without) cam belt

[Simon Jones]

There seems to be some confusion between the ability to perform a test and diagnosis you can make from the results. It's like saying you can't perform a pressure test on a coolant system that has a leak or head gasket failure. The fact that it fails the test by not maintaining the pressure is proof of the presence of a leak.

As long as the crankshaft rotates, you can perform a compression test even with a broken cambelt. The result will be no compression in one or more cylinders (as Franco says) which demonstrates a valve not working (stuck open/closed, damaged valve/seat) or problem with the piston/rings/bore.

If the test shows a consistent healthy compression on all 4 cylinders, you can pretty much eliminate a mechanical fault, and start to investigate problems with fuel and ignition systems.

[mikeWalsall]

If the cam belt as broken then the cam will not be rotating .. it possible that several of the cam lobes are keeping various valves open .. wizzing the crank over will only show compression on the 'shut' valve cylinders ..

I suppose if the piston is not hitting any valves you could manually turn the camshaft over .....

[teenmal]

I am a bit confused here, why would anyone test or Try to test the compressions on an engine that has a Broken/Snapped Timing Belt.

[Simon Jones]

I am a bit confused here, why would anyone test or Try to test the compressions on an engine that has a Broken/Snapped Timing Belt.

Deja vu
Why would it be an idea to perform a compression test? To diagnose if the cambelt has snapped. All we know is the engine just stopped:

..the engine just died as it was travelling. husband pulled over immediately, and it was duly towed to our very reliable local garage. Mechanic has had a quick look today, but initial thoughts are that it is very bad - possibly the cam belt has gone...

The cambelt has been suspected as being the cause, but not (to the best of our knowledge) confirmed. There are several other causes of the engine stopping including no fuel and no spark. These are both relatively easy to test so probably the ones to eliminate first.

Will performing a compression test do any damage if the belt has snapped? No, even if it were an interference engine (which it appears is not the case), the damage would have been done when it was running at several thousand RPM, so spinning it over on the starter can't make it any worse.

A compression test tells you to what degree the valves are working. The results will be:

a) No compression on one or more cylinders means the valves are not working: indicative of cambelt failure
b) Good compression on all 4 cylinders: cambelt is fine
c) Poor compression on all 4 cylinders: cambelt may have slipped a tooth
d) Engine wont turn over: need a new engine.

Here's details of how to test it:

http://www.denlorstools.com/autoblog/20 ... ming-belt/

[teenmal]

There seems to be some confusion between the ability to perform a test and diagnosis you can make from the results. It's like saying you can't perform a pressure test on a coolant system that has a leak or head gasket failure. The fact that it fails the test by not maintaining the pressure is proof of the presence of a leak.

As long as the crankshaft rotates, you can perform a compression test even with a broken cambelt. The result will be no compression in one or more cylinders (as Franco says) which demonstrates a valve not working (stuck open/closed, damaged valve/seat) or problem with the piston/rings/bore.

If the test shows a consistent healthy compression on all 4 cylinders, you can pretty much eliminate a mechanical fault, and start to investigate problems with fuel and ignition systems.

This cannot be done, you are trying to test a 4 Stroke engine that only has a Single stroke operation.The engine wont be able to intake any volume to be compressed (real time), the valves will be stuck closed.

[mikeWalsall]

A two stroke relies on two forms of compression .. primary (crankcase) compression and secondary .. (piston / combustion chamber) ..

To test the primary the air is sucked out of the crankcase then the retention time is measured to see how long the vacuum gauge takes to drops to zero ..

I suppose the system (it's actually a medical Penis enlargement tool) could be utilised in a sparky hole to see if it will retain vacuum ..

[Simon Jones]

This cannot be done, you are trying to test a 4 Stroke engine that only has a Single stroke operation.The engine wont be able to intake any volume to be compressed (real time), the valves will be stuck closed.

Q.E.D. By performing the test & getting a result of no compression you have successfully diagnosed the cambelt is broken. Job done

[mantaJim]

Can anyone confirm if the V6 is an "interference" engine.

Based on my experience I would say not. My engine has stopped twice - once from a failed tensioner and then a second time shorltly after repair from a sheared pulley bolt. Didn't appear to do any valve damage and has been driven regularly since last October.

Cheers

Jim

[mikeWalsall]

Dunno how close to the Bongo V6 is to the Mazda V6 626 .. but from Google, the 2.5 626 is .. not .. an interference fit engine ..

If.. it is, then maybe if the belts break you have a good chance that the valves won't hit the pistons ..

http://forum.miata.net/vb/showthread.php?t=242634

[mantaJim]

Thanks Mike

[aliwarbo]

Well - it is fixed! Got it towed to Wheelquick this morning, cambelt had snapped (although had only done 10,000 miles on it) and it was fixed within the hour with a new one on, and an oil change to boot! Just sorry we didn't get it towed there first off as we had to pay for the second journey. But all things considered, it is a small price to pay compared to having to buy another engine!

Thanks again to you all for your help and pointing us in the right direction, and the guys at Wheelquick who I have to say are all worth their weight in gold. Our van will soon be available from Wheelquick for sale as we are part exing it in mid August!

[Simon Jones]

Really pleased to hear it's all sorted and relatively painless too. Out of interest, do you know when the cambelt was last changed? They should be good for 5 years or round about 60,000 miles (whichever is soonest). With Bongos being somewhere between 8 and 18 years old, they should all be on at least their 2nd to 4th belt now.

I've had a cambelt fail on a Fiesta which was less than two years old, so you can just get the odd duff one.

[aliwarbo]

we bought the van less than 3 years ago, it only comes out in the summers and we have done about 10k miles in it since buying. it was bought from Wheelquick in Wigan who I know did put a new one on before we took delivery, they double checked it when we collected it and I have no reason to doubt their excellent reputation in the world of Bongos. So I am guessing maybe it was just a duff one? We have been unlucky but of course had it been any other vehicle than a 2L Bongo things would have been very much worse!
Picking him up in the morning and off to Anglesey for the weekend - so pleased to be getting him home as I did think he would be heading for the scrapyard a couple of nights ago!