| Author |  Topic  |
john-ayr
V4 Beginner
60 Posts |  Posted - 26 Aug 2025 : 11:39:58  
| Hi all,
I feel I have a problem with compression. My V4. Shows 110 P.S.I. on three cylinders, and 90 P.S.I. on one, Engine hot. Does anyone know if this is a reasonable discrepancy? If I put a little oil down the cylinder, it goes to 110 P.S.I. is this a ring problem?
The engine is a rally spec., about 1700 c.c. on DCOE 40's
John-Ayr |
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SAAB96L
V4 Mad
United Kingdom
749 Posts | Posted - 26 Aug 2025 : 18:18:30
| Hello John,
That does appear to be a little down and most likely to be the ring. It will be useful to do the compression test from overnight cold and see what those readings are.
The fact the compression comes back to 110 certainly indicates a ring issue, as the oil is helping to form a temporary seal on the defective ring.
TBH, I would expect the compressions to be generally higher than 110 overall.
Regards.
Richard |
Edited by - SAAB96L on 26 Aug 2025 18:21:14 |  |
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john-ayr
V4 Beginner
60 Posts | Posted - 27 Aug 2025 : 19:54:32
| Richard,
many thanks for responding.
The cold figures would appear to be somewhat higher, which surprises me.(95 psi on the low cylinder, 125psi on the other ones)
This has the look of a winter project in a cold garage!
Regards, John D. | |
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Wheelspin
V4 Fan
Denmark
105 Posts | Posted - 28 Aug 2025 : 10:48:02
| Hi John.
If compression goes down on a warm engine it usually means your valve clearances are too small. This is quite normal when running unleaded petrol on original heads without hardened seats. Lead replacement additive unfortunately only delays this issue and don’t prevent it completely.
This does however not explain the one low cylinder, so could be worn or stuck rings.
Regards, Mads | |
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john-ayr
V4 Beginner
60 Posts | Posted - 28 Aug 2025 : 20:05:02
| Mads,
thanks for your input. I do not know if the heads have hardened seats, Should I increase the valve clearances then?
Regards John D. | |
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melle
V4 Guru
United Kingdom
4145 Posts | Posted - 29 Aug 2025 : 10:11:59
| If the heads haven't been rebuilt they wouldn't have hardened seats. Valve clearances should be the same, but you may need to adjust them at shorter intervals. Checking valve clearances never harms, but this sounds more like rings to me. A healthy V4 should show around 150psi ±10 on all cylinders, I would think.
www.saabv4.com | |
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john-ayr
V4 Beginner
60 Posts | Posted - 29 Aug 2025 : 19:11:33
| Melle, thanks for that.
I contacted the owner that had work done on the engine. He tells me that it is of 1740c.c., forged rods and pistons. The head also got bigger valves, which would suggest that they did the hardened seats, having done all the other work.--- It's looking like rings, which I suppose is engine out.
Regards John D. | |
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SAAB96L
V4 Mad
United Kingdom
749 Posts | Posted - 31 Aug 2025 : 10:19:10
| Hello John,
If you want to narrow down the issue you have, then I would suggest carrying out a Cylinder Leak Test - CLT
This procedure allows you to diagnose where the issue lays by pressurising each of the 4x cylinders individually - of course you have identified the weak cylinder, but not the actual issue. In addition the test can be performed with the engine in or out situ and solely requires the spark plugs to be removed and both the inlet + exhaust valves closed - basically TDC.
The breakdown of the test would be as follows:
1- A ring issue will allow compressed air to be circulated out through the rocker cover crankcase breather pipe and also the dipstick.
2- Head gasket issue will show up as bubbling through the rad and expansion bottle
3- Inlet valve issue [not seating] you will have air escaping out through the inlet manifold
4- Exhaust valve issue [not seating] you will have air escaping through the exhaust tail pipe.
All of the above, depending which is the issue, will be highly prominent and identifiable.
I should also add that the CLT kit has a pressure gauge which indicates if there is a SIGNIFICANT pressure loss. There will always be a gradual minor loss of pressure as there is of course a microscopic gap between the piston ring and cylinder wall - by design of course. The cylinder set-up in good form, will maintain a considerable amount of pressure with NO discernible drop in pressure over twenty to thirty seconds.
These kits are not expensive and I use Draper; here is an example: https://www.toolden.co.uk/p/draper-35884-cylinder-leakage-kit-7-piece/?gad_source=1&gad_campaignid=17349046775&gbraid=0AAAAADfua22aJ1SX57X4-VglGk7iraO6w&gclid=Cj0KCQjw5c_FBhDJARIsAIcmHK9kM3JWBBedwrFm4XzejVk1q-sMw7azBYqiQhGiSAiDiTPSMvrpgLAaAnoeEALw_wcB#description
You could of course, take the car to a garage and they will be able to carry out the procedure for you. The whole test can be done within the hour.
At the conclusion of the test, you will know exactly what you are dealing with and can plan onwards from there.
Hope that helps ??
Regards.
Richard.
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Edited by - SAAB96L on 31 Aug 2025 10:24:26 | |
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john-ayr
V4 Beginner
60 Posts | Posted - 02 Sep 2025 : 12:01:25
| Richard,
have been away for a few days, just got back to the site.
Many thanks for your most detailed instructions regarding my engine problems. I think I shall get one of these kits and tackle this myself,--- we don't seem to have garages round here that will do this kind of thing.
Thanks again,
Regards, John D.
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SAAB96L
V4 Mad
United Kingdom
749 Posts | Posted - 02 Sep 2025 : 16:20:23
| No problems, John.
Probably obvious, but you will need a compressor to use the equipment.
Regards.
Richard. | |
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john-ayr
V4 Beginner
60 Posts | Posted - 05 Sep 2025 : 16:23:47
| Richard,
yes, I don't have one ,but I know a man that does.
Before I go there, when the engine is idling, there is quite a lot of air pressure evident from the cam box filler. If the problem is rings, I can't believe they have all become defective at the same time. Perhaps they have become gummed up through lack of use.
Do you know of any product that would help to un-gum them? Regards John D. | |
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SAAB96L
V4 Mad
United Kingdom
749 Posts | Posted - 05 Sep 2025 : 16:47:36
| Hello John.
I always use Redex every couple of fuel fill-ups, this helps to keep gumming up of engine parts, cylinder head, carbs and fuel lines clear or certainly a lot healthier than if not using it. Best place to purchase is Tesco etc as they always seem to sell at 50% off !!
If your rogue cylinder does have a ring issue [which I believe it does] then it may be bad enough to allow excess blow-by. There will always be a bit of pressure due to crankcase breathing and more so in a closed system.
In the past, owners have put a glug of Redex straight down the carb mouth to help high impact cleaning BUT the smoke emission by doing this as the Redex is burnt off is quite substantial !! It may be frowned upon by environmentalists and create a similar scene as to the laying down of smoke during the Battle of Jutland !!!
I use Redex on all 3x of my classics have done so for years, personally I believe it helps immensely by keeping the engine and fuel induction system gum-free.
This explanation is also of interest and refers to clogging up of the PCV and its hoses which the SAAB V4 most certainly has:
Excessive crankcase pressure is primarily caused by a malfunction in the crankcase ventilation system, which prevents the expulsion of blow-by gases. Common causes include stuck or worn piston rings that increase blow-by, a clogged or faulty PCV (Positive Crankcase Ventilation) valve, and blockages in the ventilation hoses or oil/vapor separators. In forced induction engines, i.e. Turbo, high intake manifold pressure can also be forced into the crankcase, overwhelming the PCV system and raising pressure
Regards.
Richard. |
Edited by - SAAB96L on 05 Sep 2025 16:57:45 | |
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john-ayr
V4 Beginner
60 Posts | Posted - 07 Sep 2025 : 23:32:34
| Hi Richard,
redex I shall certainly use. The P.C.V. I had not considered and will check that out.
One other thing, if I have to draw the pistons, rather than take the engine out, I am a bit short of space in my garage, I thought I would cut a rectangle out of the bodywork under the sump, drop said sump and weld it back in after the job. (I can hear the howls of anguish even as I write this!!) One would have to do a similar repair if there was a rust problem in this area, I suppose. What do you think?
Many thanks again, John D. | |
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melle
V4 Guru
United Kingdom
4145 Posts | Posted - 08 Sep 2025 : 07:52:27
| I think you'd be making things rather more difficult for yourself doing it that way.
www.saabv4.com | |
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SAAB96L
V4 Mad
United Kingdom
749 Posts | Posted - 08 Sep 2025 : 16:06:36
| I'm with Melle.
If the engine does need to come out which I suspect will be the case, it's an ideal time to check the engine over thoroughly. In addition to that, addressing any other issues that may be apparent but difficult to see & treat with an engine in situ.
I am howling !!
Regards.
Richard. | |
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john-ayr
V4 Beginner
60 Posts | Posted - 08 Sep 2025 : 16:11:32
| Oh no, outvoted by experts!!
John D. | |
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| Topic | |
Compression.
