QuestionQUESTION: Dear Barrie,
I have MGB with the engine 18V 673Z which was renovated last year. I had a lot of problems concern quality of work with the company but finally after one year the engine was done. I’m not sure what exactly the company did with the engine but I can say that rings and acetabulums are new and new funnels was installed. I was driving car this year and did around 2000 miles. Unfortunately I have problems with start-up the engine when is cold and with the temperature which is rice fast up to 212 Fahrenheit during driving the car acceleration is low as well. I did the engine compression test and received following numbers 1-137 PSI,2-133 PSI,3-119 PSI,4-133PSI with the valve’s slit ,0015’’. The wet test shows better compression which are 1-158 PSI, 2-148 PSI,3-133PSI,4-145 PSI . The car manual says that the compression should be around 170 PSI. Could you tell what should I check, exchange or repair and what can be a cause in this case.
Best regards,
Mariusz
ANSWER: Hi Mariusz
The actual pressures do not matter so much as the variance between them. A figure of 170 could only be achieved with a compression ratio of more than 11:1, and yours should be 9.9:1 according to my manual, so the pressures are about right.
The general rule with compression testing is that cylinders should not differ by more than 10psi.
From your figures, I see that the dry readings are about 10-12 psi less than the wet ones, and that cylinder 3 is a bit down, both dry and wet. This tells me that the rings have not yet fully bedded in, and that you have a problem with cylinder #3.
Because the wet figure for #3 is also down a bit, you may have the beginnings of a problem with the head gasket, or an inlet valve that needs re-seating. I suspect that cylinder #3 is venting into the water jacket and blowing steam into the cooling system. The sender for the gauge normally sits in hot water, but if a bubble of steam goes by, that could cause it to shoot up to 212F.
I see you have a new engine with 2,000 miles on it. Did you re-torque the cylinder head after the first 500-1,000 miles? If not, that could be the problem.
Before you take the head off, get a garage to do a pressure test on your cooling system. Also, if they have the equipment, get them to pressurise cylinder #3 through the spark plug hole with the valves shut, and listen where the air comes out. If it bubbles up through the radiator cap, then you definitely have a failed head gasket.
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QUESTION: Hello Barrie,
Thank you for your detail answers. I decided to took off the head from the engine and noticed that you were right! There was a lot of black coke on the gasket and engine block between cylinder no. 2 and 3.So, I did the next test.
I poured kerosene in to each cylinders and in to head. Unfortunately I found that cylinder no. 1 and 4 are leaky(cylinder no.3 and 4 are tight). The kerosene gone after half an hour. The surface of the cylinders are not smooth and I can see that cylinder caught the piston rings a few times. I can feel unevenness by touching cylinder by finger as well. I don't know why it happened. The cylinder head and valves are tight.
I'm not sure concern require engine compression but I found in "Owner workshop manual" by Haynes that the engine compression should be 11,9 kgf/cm or 170PSI. What kind of compression should have engine 18v673z after reconditioning and what is the difference between low compression and height compression? May be they write about high compression.
I'm going to level the cylinder head and exchange the head gasket but I don't know what to do with the pistons and cylinders. Do you think that I have to change pistons rings and level of the cylinder no. 1 and 4.?
Could you explain me what does it means "re-torque the cylinder head".
Please let me know. Thank you very much.
Mariusz
AnswerHi Mariusz
Like I said before, the actual compression figures do not matter. What matters is that they are all within 10psi. If you keep turning the engine over, the reading will continue to climb, so it is best to turn the engine for the same number of compressions. You can hear them, and I usually do 5 `pumps' on each cylinder.
The cylinder head nuts must be tightened in sequence, starting with the middle two and working outwards. The sequence is shown in your Haynes manual. In my manual it is on page 12, diagram 1:3. You will need a torque wrench. First tighten all the nuts to 30 ft-lb, then tighten them all to 45 ft-lb. After the engine has been run for half an hour, you should take the rocker cover off and re-torque the nuts to 45 ft-lb and re-set the tappets. After 500 miles you will need to do it all over again for the last time.
When your engine was renovated, they probably bored it and fitted oversize pistons. Before fitting the pistons, they should have honed each bore, leaving a cross-hatch pattern on the bores. This helps the piston rings to bed-in. You can feel this cross-hatching with a finger nail. You can probably see a wear pattern starting to happen as the rings polish away the cross-hatching. Cylinders 1 and 4 often polish quicker than 2 and 3. This is normal, and they will all become nice and shiny eventually, but it may take 10,000 miles.
