Compression test results. Gutted!
Discussion
Did a compression test and got the following:
1) 170 2) 170
3) 165 4) 160
5) 175 6) 145
7) 175 8) 165
So I'm pretty gutted to say the least! Syringed a fair bit of oil into cylinder 6, did the test again and got 175, so I have a ring problem right? It also smokes when you rev it up, and all the plugs soot up quickly, and it misfires. Engine sounds good otherwise.
I've changed engines, gearboxes, clutches, camshafts, cambelts etc before but never rebuilt an engine. I do have the equipment; engine crane, and engine stand and double garage, but am I going to regret attempting this? What's it going to cost me doing it myself?
Any links to a good guide and/or rebuild kit?
1) 170 2) 170
3) 165 4) 160
5) 175 6) 145
7) 175 8) 165
So I'm pretty gutted to say the least! Syringed a fair bit of oil into cylinder 6, did the test again and got 175, so I have a ring problem right? It also smokes when you rev it up, and all the plugs soot up quickly, and it misfires. Engine sounds good otherwise.
I've changed engines, gearboxes, clutches, camshafts, cambelts etc before but never rebuilt an engine. I do have the equipment; engine crane, and engine stand and double garage, but am I going to regret attempting this? What's it going to cost me doing it myself?
Any links to a good guide and/or rebuild kit?
A leak down test is far more accurate for diagnosis but it would seem to initially point toward the rings/bores.
A worn cam can affect the readings too.
Whats the history of the engine, it may just benefit from a thorough service and some use if its just been sat doing nothing.
A worn cam can affect the readings too.
Whats the history of the engine, it may just benefit from a thorough service and some use if its just been sat doing nothing.
Edited by Belle427 on Saturday 25th November 18:23
Edited by Belle427 on Saturday 25th November 18:33
Belle427 said:
A leak down test is far more accurate for diagnosis but it would seem to initially point toward the rings/bores.
A worn cam can affect the readings too.
Whats the history of the engine, it may just benefit from a thorough service and some use if its just been sat doing nothing.
Car has no history (bought it cheap at auction). All I know about it is, according to MOT history, it's only done about 500miles since 2007! Basically it went to the MOT station and back for 16 years. The engine oil looks new, but stinks of fuel/exhaust.A worn cam can affect the readings too.
Whats the history of the engine, it may just benefit from a thorough service and some use if its just been sat doing nothing.
Edited by Belle427 on Saturday 25th November 18:23
Edited by Belle427 on Saturday 25th November 18:33
I've done about 100 miles in it since obtaining it about a month ago. It drives nicely, apart from the misfire.
V8covin said:
145 psi isn't that low is it.......and if all your plugs are fouling up doesn't that point to it running too rich ?
Does it have a lambda sensor?
Pleased to hear you don't consider it that low. Yes, the rich running is probably not necessarily related which is something I need to investigate. No Lambda sensors, since this is a precat.Does it have a lambda sensor?
145PSi sounds pretty low to me. My 350i Wedge is around 200PSI on all cylinders. When I had a leaking head gasket into the water jacket, the offending cylinder still read 175PSI!
If the engine has been standing for a long time it could just be stuck rings. I believe putting some diesel in the cylinder bore and letting it soak for a day or two is the usual cure?
If the engine has been standing for a long time it could just be stuck rings. I believe putting some diesel in the cylinder bore and letting it soak for a day or two is the usual cure?
Do all suggested above.
175 is not bad so maybe you can indeed free up clogged rings and compression improves with use.
I’d add injector cleaner into the fresh fuel as a good measure.
Then I’d change the oil before running it.
What’s it going to cost you.
How longs a piece of string.
Ideally you have a good engine with one pot low and all others within tolerance so a new set of gapped ( expert skills required ) rings in that pot might fix it which would be a relatively cheap job but I’m sure most engine builders would also recommend honing at least the bores your replacing the rings in which then suggests you simply re hone all the bores and add new gapped rings to all of them.
This is an excellent way or refreshing the compression in an otherwise standard bore size engine.
So if you are forced to go this far a short block rebuild including new shells etc for bottom end by a reputed builder makes sense because they will do all this. If your happy checking end float etc then rebuilding the engine yourself is always an option but machining and parts costs add up and a short block refresh rather than full rebuild by a good builder who knows what does and does not need changing is worth the bit extra it costs IMO.
It might end up just being a poorly seating valve or broken spring/ cam or timing/ ignition issue so do all your basic checks first.
Caveat
When people see the price of rebuilds via TVR specialists this usually includes removing and replacing the engine as well as rebuilding it.
If you doing much of the donkey work you can get your engine refreshed at a reduced cost. Good TVR specialist find all the other s
t that’s wrong with your car and insist it’s repaired which is a good thing if dammed expensive so be prepared.
If your handy and feel sure the engine needs work I’d be inclined to drop the sump and check the big end shells on at least the one low pot to investigate crank condition as that’s going to add cost if it’s showing wear.
175 is not bad so maybe you can indeed free up clogged rings and compression improves with use.
I’d add injector cleaner into the fresh fuel as a good measure.
Then I’d change the oil before running it.
What’s it going to cost you.
How longs a piece of string.
Ideally you have a good engine with one pot low and all others within tolerance so a new set of gapped ( expert skills required ) rings in that pot might fix it which would be a relatively cheap job but I’m sure most engine builders would also recommend honing at least the bores your replacing the rings in which then suggests you simply re hone all the bores and add new gapped rings to all of them.
This is an excellent way or refreshing the compression in an otherwise standard bore size engine.
So if you are forced to go this far a short block rebuild including new shells etc for bottom end by a reputed builder makes sense because they will do all this. If your happy checking end float etc then rebuilding the engine yourself is always an option but machining and parts costs add up and a short block refresh rather than full rebuild by a good builder who knows what does and does not need changing is worth the bit extra it costs IMO.
It might end up just being a poorly seating valve or broken spring/ cam or timing/ ignition issue so do all your basic checks first.
Caveat
When people see the price of rebuilds via TVR specialists this usually includes removing and replacing the engine as well as rebuilding it.
If you doing much of the donkey work you can get your engine refreshed at a reduced cost. Good TVR specialist find all the other s
t that’s wrong with your car and insist it’s repaired which is a good thing if dammed expensive so be prepared. If your handy and feel sure the engine needs work I’d be inclined to drop the sump and check the big end shells on at least the one low pot to investigate crank condition as that’s going to add cost if it’s showing wear.
Edited by Classic Chim on Sunday 26th November 10:14
I would just drive it as much as possible, ideally over some distance (maybe a track day ?) to get it really warm after a basic service as already mentioned. A miss fire can be anything.
A contaminated Air Flow Meter (AFM) for example gave me a terrible miss fire. Try just unplugging it and got for a drive to eliminate.
If it’s not been used, check ALL of the fuel lines including the short lengths by the tank / fuel filter
A contaminated Air Flow Meter (AFM) for example gave me a terrible miss fire. Try just unplugging it and got for a drive to eliminate.
If it’s not been used, check ALL of the fuel lines including the short lengths by the tank / fuel filter
Edited by ESDavey on Sunday 26th November 17:46
Edited by ESDavey on Sunday 26th November 17:47
Sorry to hear your woes, in my opinion you have a few things to consider. From the test you have carried out, you likely have an issue with the rings or the bores. Not easy to fix in situ, so it’s engine out time. With the engine on the bench how far do you go? If the engine has covered anything near 50k changing the cam would be a good idea, I would want to change the oil and water pumps, then there are valve guides and seals to consider. Then new rings maybe a re-bore and change the bearing shells?
You don’t need a TVR specialist just a competent garage.
If your skills are good enough, get an engineering shop to check the tolerances and do the work yourself. Cheaper and much more rewarding.
Good luck Lee
You don’t need a TVR specialist just a competent garage.
If your skills are good enough, get an engineering shop to check the tolerances and do the work yourself. Cheaper and much more rewarding.
Good luck Lee
OP you may be over-thinking this, piston rings get gummed up when motors have been stored, sometimes you just need to put some mileage on it.
Had just the same with an old 750/4 motorcycle, cr readings all over the place - a shot of redex and a couple of tanks of fuel through it then repeated the compression test and everything back within spec.
how many miles has the car been driven in the last year?
how many miles in one go was the car last driven ?
whats the milage of the engine?
has the cam ever been changed ?
compression test has been done with full-open throttle?
if the car only stood around, you might be lucky that its just a sticky pistonring---so drive the car for at least 150-200miles in one go
if the cam has never been replaced and the mileage is above 40-50k...it might be a worn cam
compresstion test results ,as pure value, is not so important...important is that they are about equal.
depending on the cam fitted (overlap etc) a healthy engine can show a lower reading as the same engine with a standard cam (less overlap).
your lowest is 10bar...the highest 12 bar...i would be more concerned if the lowest would be in a region of 6-8 bar.
so for me nothing too unusual or concerning.
drive the car and report if things get better.
how many miles in one go was the car last driven ?
whats the milage of the engine?
has the cam ever been changed ?
compression test has been done with full-open throttle?
if the car only stood around, you might be lucky that its just a sticky pistonring---so drive the car for at least 150-200miles in one go
if the cam has never been replaced and the mileage is above 40-50k...it might be a worn cam
compresstion test results ,as pure value, is not so important...important is that they are about equal.
depending on the cam fitted (overlap etc) a healthy engine can show a lower reading as the same engine with a standard cam (less overlap).
your lowest is 10bar...the highest 12 bar...i would be more concerned if the lowest would be in a region of 6-8 bar.
so for me nothing too unusual or concerning.
drive the car and report if things get better.
Edited by LLantrisant on Friday 29th December 11:16
Gassing Station | Griffith | Top of Page | What's New | My Stuff