• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Compression test results

Share this page

Creatrixx

Creatrixx

Jedi Hopeful
Offline
In trying to find out why my mini is still overheating, we did a compression test. We thought the numbers were kinda low, but there wasn't a serious spread. They ranged between 115 and 95 psi. Is that really bad? What could that mean/how do I fix it? My boyfriend suggested that the rings could be worn.

Thanks!
Melissa the n00b
 
When you run the compression test, make sure the engine has reached operating temperature. While you take the measurements you have to hold the throttle wide open and crank until the reading no longer increases.

The target is to have (hopefully) all the values within about 10% of each other. It's not that there's a specific target number to aim for. In your case, you could make the argument that an "average" number was 105 psi and therefore, both your high and low numbers were almost within 10% of the average. Your numbers may be a little low, but BL/Rover did make some low compression engines so I can't say if your number are "too low".

You and your boyfriend may wish to invest in (or borrow) a leak down tester. (That assumes you already own or have access to an air compressor. ) You can go to eBay and search for leak down tester to see pictures of them. (In fact, I just checked and there's a guy from TN whose selling what look to be the Harbor Freight leak down testers for very low prices. ) To use a leak down tester you bring a piston to top dead center connect the tester's spark plug adapter to that cylinder. You apply 100psi air from the compressor to the tester and read its output gauge which gives you a "percent" leakage. You WILL have some leakage. While the tester is blowing air into the combustion chamber you listen around the car for where the air is escaping (out the tailpipe = exhaust valve leak, out the carbs = intake valve leak, into the radiator OR out an adjacent spark plug hole = head gasket blown, out the dipstick hole = blow by past the rings). Some air WILL go by the rings. If you see a leakage percent below 20% and it's coming out the dipstick hole, I would not worry about the rings yet.

So, did you get that radiator and pump installed yet? I wouldn't worry about the compression and its affect on temperature until you've got that installed.

BTW, did you check your ignition timing to see if it is correct? Timing can also affect the operating temperature of the engine.
 
What about the temperature sending unit?

Did you replace the old one with a new one?
 
minimotors seems to be a bit flaky. I haven't gotten the radiator from them yet and I can't get in touch with anyone there. It also took a week for them to answer an email before. So, no, no radiator installed yet. I had the timing adjusted at a british repair shop, but I haven't checked it personally. I'm just trusting them. We did, however, discover that it was running lean when we checked the compression so we adjusted that.

I found that at night, if I go 45 my car stays very cool. I discovered this last night and thought that my gauge broke and the needle was just hanging straight down. So, this gives me hope that the radiator and pump will actually resolve my problem. Hopefully I can get it soon enough to get it installed before the pittsburgh vintage grand prix which is a 4 hour drive away at actual highway speeds...

I'll look into the leak tester. Thanks!
 
Regarding Ron's comments, Mini temperature gauges are a problem. According to a friend of mine, there are at least three different sending units with a corresponding different number of gauges. The problem is, no one knows which sender matches which gauge. A number of people I know have given up and installed a mechanical gauge with capillary tube instead of using the electric gauge. The Smiths ones can be expensive but the generic $20 Suntune/Actron ones from Advance Auto will work just fine... they just won't match your other gauges.

You may also want to invest in something like a small infrared (non-contact) thermometer. You can buy them at Radio Shack and now Harbor Freight has started carrying them. If you get the car up to temperature and aim the thermometer at a black painted portion of the radiator you'll have a rough idea (probably within about 5 degrees) of what the coolant temperature really is.
 
The gauge does read slightly hot and we have used an infrared tester to find that out (we aimed it at the top radiator hose, though I could imagine that the less insulated top part of the radiator may be a little more accurate).

Would having the temp sending unit be off actually cause any problems other than the gauge being a bit off?
 
The gauge or sender being out of calibration won't cause any problems with the car. It's just annoying for the driver.
 
Back
Top