Poor Compression?!

Submitted: Monday, Mar 22, 2004 at 16:48
ThreadID: 11461 Views:1889 Replies:9 FollowUps:9
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Posted: Mon Mar 22, 2004 8:22 am Post subject: Poor compression

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I was at a 4WD training session this weekend, and one of the things we did was a stall start. I drove up the hill, stalled the car with the brakes, put it into reverse, and let go of the brakes. Now, the instructor told me that the compression of the engine should hold the car back, but mine was "ticking" backwards, like rolling thorugh one cog, taking a break, then the next cog. They attributed this too poor compression. None of the other cars did it, and it wasnt meant to happen to min either.

What does this mean? Do I have to do anything about it? Should I be concerned?

I drive an old HJ47 troopy with a 2H engine (340000km) diesel...

Thanks,

Jo
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Reply By: Davoe - Monday, Mar 22, 2004 at 17:20

Monday, Mar 22, 2004 at 17:20
2h should have as high compression as anything else so could be a bit of a concern. Look for other sighns such as blowing smoke hard to start and general poor performance. At 340k it should be starting to get a bit tired but may well have another 200k left. If it goes ok keep it going as a rebuild would not be cheap. Take it to a mechanic and get it compresion tested then you will be in a better position to consider your options. I have a 2h service manuel at home and would be happy to give you the acceptable ranges for compression if you want.
Just read the rest of your q compression is the pressure built up in the head of the motor when the piston rises. Like putting your finger over the end of a syringe (no needle) and pushing the plunger and compressing the air. This compression is the lifeblood of the diesal engine as it is the compression along with heat that burns the diesal instead of the spark plugs in a petrol- low compression = poorly burning fuel. 1 last thing dont let a mate try measuring the compression with a standard gauge as diesals have 2wice the compression of a petrol , not good
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Reply By: Wayne (NSW) - Monday, Mar 22, 2004 at 17:55

Monday, Mar 22, 2004 at 17:55
Jo,
340,000 km the rings or valves might need looking at but you have had a good run. I have had some petrol motors do the same thing and they didn't have the km's that your motor has.

When doing a reverse stall start don't take you foot off the brakes completely. Allow the bulk of the weight of the vehicle to be taken up by the gearbox but at the same time using the brakes as a back up.

Wayne
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Reply By: Rosco - Bris. - Monday, Mar 22, 2004 at 18:01

Monday, Mar 22, 2004 at 18:01
Jo

I wouldn't be overly concerned. From the way you described it there was a pause between "ticks".

The vehicle was being held by the compression in 1 cylinder only, for quite a while. So it's fair to expect the compression to sneak out past the rings ... until it lets go and then the next cylinder takes up and so on and so on.

Hence you can expect it to lug back down the hill a bit at a time. If you've got the time it's a fairly safe way to descend.

Cheers
AnswerID: 51441

Reply By: Martyn (WA) - Monday, Mar 22, 2004 at 22:48

Monday, Mar 22, 2004 at 22:48
Jo,
How steep was the hill? How much oil do you burn? If the vehicle isn't showing signs of being near its end of life cycle get a few basic checks done, compression test is one check but as mentioned don't use a standard guage the results could be a tad frightening, its nearer three times the compression pressure a GQ is around the 3000 kpa mark where as a normal petrol engine is around the 1000 kpa mark, be careful. The compression test will give you an idea where to start, if the engine backfires on the overrun take a look at the valves, could be cracks in the head, this list is long and varied.
If the hill is too steep you could always select low range, if it rolls back in low range either the tyres are slipping over the surface or you've got holes in your pistons.
I wouldn't put 340k as an extraordinary mileage. Hope this helps, my opinions as usual.
Keep the shiny side up

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Follow Up By: Wayne (NSW) - Monday, Mar 22, 2004 at 23:55

Monday, Mar 22, 2004 at 23:55
Martyn,

All stall starts should be done in low range. Some vehicles will still tick over even in low range, that is why the brakes should never be fully released.

Wayne
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Follow Up By: Martyn (WA) - Tuesday, Mar 23, 2004 at 22:05

Tuesday, Mar 23, 2004 at 22:05
Wayne,
I stand corrected, but on sand dunes when just didn't make it I've used high range to get get better control when reversing back down again, I suppose in some cases it varies with experience and also the situation. Point taken
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Follow Up By: Wayne (NSW) - Wednesday, Mar 24, 2004 at 00:16

Wednesday, Mar 24, 2004 at 00:16
Martyn,

Sand driving is diffrent again. Momentum is required, and that means high range 4WD. If going up a sand dune and you don't make it you are useally bogged and not going any where, selecting reverse gear and driving down is not a problem. If on a steep hill however as soon as the clutch is used and the vehicle not stalled, then it has a chance of running away. On these hills low range would be selected befor trying to drive up the hill
The only time I ever use low range on sand is if I am bogged and need to rock back and forward to pack down the sand and when a bit more power is required.
We go up to Stockten Beach, Newcastle, and spend a day there playing on the dunes. We are there all day and don't have to drive over the same sand dune twice. Good practise area for the Simpson.

Wayne
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Reply By: ross - Tuesday, Mar 23, 2004 at 02:36

Tuesday, Mar 23, 2004 at 02:36
Maybe the instructor is someone who has always owned new vehicles and cant believe an old 4x4 can give good service.As the toopie didnt run away I wouldnt be too worried. Even compression on each cyl is more important than high compression(within reason).
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Reply By: Member - Jo (NSW) - Tuesday, Mar 23, 2004 at 07:27

Tuesday, Mar 23, 2004 at 07:27
The hill was probably about 30 degrees, and I was in low range. I burn about 3 liters per 5000km, and it doesnt smoke, its easy to start and runs fine as far as I know. I wouldnt have thought anything was wrong unless I had done that hill stall start, but it might still be fine even! Ill get a compression test done, and post the results back here for a second opinion. Thanks guys!

Jo
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Follow Up By: Davoe - Tuesday, Mar 23, 2004 at 11:01

Tuesday, Mar 23, 2004 at 11:01
As promised the compression figures should be
Desirable- 2746kpa
minimum- 1961kpa
maximum variation between cylinders 196kpa

Make sure a compression test is done without oil forst then with a bit of oil dropped down the cylinder. If a low reading is achieved initially but picks up after the adition of oil that indicates valves rather than rings. If this is the case the first port of call is valve adjustments- after this it gets expensive. Not all doom and gloom though if it starts ok and runs fine cant be to bad. compression test should be around 1 hrs labour which is cheap enough as it will tell you plenty about the condition of the motor and give you an informed choice about what to do keep sell or rebuild
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Follow Up By: Member - Nino F (VIC) - Tuesday, Mar 23, 2004 at 23:04

Tuesday, Mar 23, 2004 at 23:04
Davoe,
If the compression picks up after the additon of a few drops of oil
( a "wet" test. ) this indicates worn rings. The oil seals around the top of the pistons/cylinder walls and not the valves.
cheers...Nino
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Follow Up By: Davoe - Wednesday, Mar 24, 2004 at 11:02

Wednesday, Mar 24, 2004 at 11:02
Nino f is right whoops! i was just checking to see if any body actually reads my responses honest! should have been if the compresion doesnt pick up after the addition of oil then it is a valve problem
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Reply By: Glenno - Tuesday, Mar 23, 2004 at 21:24

Tuesday, Mar 23, 2004 at 21:24
Could it be the clutch slipping?
AnswerID: 51649

Follow Up By: Member - Jo (NSW) - Tuesday, Mar 23, 2004 at 21:55

Tuesday, Mar 23, 2004 at 21:55
How would I know this? Any other symptoms come with this?

Jo
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Follow Up By: Davoe - Wednesday, Mar 24, 2004 at 11:05

Wednesday, Mar 24, 2004 at 11:05
doesnt sound like a slipping clutch to me as if it was the engine would not slowly tick over and if the clutch was that bad you would not be able to take off from the traffick lights. Symptoms of a stuffed clutch are releasing the clutch and getting no/minimum drtive
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Follow Up By: Member - Jo (NSW) - Wednesday, Mar 24, 2004 at 11:09

Wednesday, Mar 24, 2004 at 11:09
It drives fine, gear changing feels fine, so not clutch then.
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Reply By: Davoe - Wednesday, Mar 24, 2004 at 13:37

Wednesday, Mar 24, 2004 at 13:37
Sounds like the old girl operates qute well from your coments. just because you sneeze doesnt mean you have a cold try the compression test at next service but I reckon you will have to wait a while for her to fall apart on you
AnswerID: 51752

Reply By: jhetland - Tuesday, Mar 30, 2004 at 16:23

Tuesday, Mar 30, 2004 at 16:23
Follow up posted to new thread to put it on the top.

Thanks,

Jo
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