cwumbs

Does anyone know if compression test pressures will increase or decrease if carried out on a cold engine as opposed to hot (cant start mine)

bse50

Hot engine: lower compression.
That's why the test should be performed on an engine at around its operating temperature.

cwumbs

how much difference do you think?

bse50

Enough to make a cold test useless. The fact is that there's a graph per RPM variation but not one for temperature differences so that's undeterminable.

RIWWP

iTrader: (2)

Heat makes things expand, our engine parts are no different. The housings will expand away from the rotor, increasing the amount of distance that the seals have to cover, and if the seals are worn, damaged, or otherwise not optimal, then the expansion generally is too great for them to cover, and you lose compression. The loss of compression from poor sealing means that there will be blowby and or blowback (if it's the apex seals, not sure how to describe the side seals, or where it goes), and this generally starts increasing the wear on the housings at a faster rate, the way water wears away at rock over time.

Hard warm starts is one of the signs of failing compression, and unlike most of the other signs of failing compression, this one doesn't have too many other explanations (failing fuel pump is the only other hard-warm-start explanation I am aware of, pump overheats, letting the car sit lets it cool off and start pumping again)

Edit:
Just re-read your post. I am guessing that you can't start when cold? That is usually electrical and/or ignition related. Weak or failed battery, starter, coil(s), plug wire(s), plug(s).

cwumbs

seems i've got a problem then, it wont start-so cant get it to operating temp-so cant perform an accurate compression test- so cannot rule out compression as the problem. But just in case somebody has any thoughts, the readings i'm getting are;
front; 78,80,77psi
rear; 90,88,87psi
crank speed;178rpm

RIWWP

iTrader: (2)

I am not sure what PSI is needed. I usually see compression test numbers here of 6.0 - 9.7 or so. Guessing that is bar?

However, if that is your crank speed via the starter, that is probably your problem. The starter needs to be able to spin the engine at least 250rpm to start, though 300-350 is in spec and ideal. If you can only spin it 178rpm, you are below stall speed.

Check your starter.

cwumbs

The engine was flooded when I bought the car and wouldn't start. rolling down the trailer ramp when offloading it I bump started it in reverse whilst hardly moving and it started-smoked a lot! after a minute it cleared. stopped it-wouldnt start on key again -bumped it again-started, kept it running for a few mins then it cut out whilst revs were at about 5000 then still wouldnt start on the key. Left it overnight on charge. started easily the next morning on the key ran for about a minute I attempted to reverse out of the garage and it cut out, then wouldnt start again so thats why I'm needing to get an accurate comp test. the engine fault light is on!

cwumbs

Forgot to say the car is an 04 with 18600miles

RIWWP

iTrader: (2)

Having a CEL (check engine light), could be alot of things, including trivial things like "your gas cap is loose". Until you have the code read, you won't know what it is.

Knowing that it is an 04 with that low of mileage does suggest several things:

1 - This car was not driven much (~3,000 miles per year)
2 - Since it wasn't driven much, it might easily have oil sludging and if it sat for a really long time (1 year +), it might have internal housing damage from the seals sitting against it in one place. Dis-similar metals start rusting.
3 - If it wasn't driven much from the beginning, it is entirely possible that it has the original starter, which was under-powered. 2004 had a TSB to upgrade the starter for free, though only if it was in bumper to bumper warranty, which it is clearly out of due to time. If you have the original starter, that would certainly explain your low crank speeds
4 - due to no-driving, your seals and/or ports could be stuck shut, which could certainly make you have trouble keeping the engine running.
5 - If you haven't replaced the battery yet, check it at a minimum, but guessing you did that.


At an absolute worst-case scenario, it is possible that one or more seals had rusted to the housing, and when you bump-started it, you left part of the seal(s) on the housing.


Get that CEL code read if you can, check the starter (there is a comparison picture around the board here somewhere on how to tell which starter you have, original or upgraded). If you have the original starter, get the upgraded one (used or reman should be fine), and see how you fare.

Let us know.

cwumbs

I've just bump started it again and drove down the road. Engine is quite erratic, slow to increase revs, it misfires and probably drops off one chamber completley- intermittently. oil pressure was about 60% of the gauges full travel. There is quite a bad "rattle" which I think is coming from the cat. I then did a compression test whilst the engine was warm--60psi on all three (should be 98psi minimum at 250rpm)) But cranking speed was only 150rpm (probably lower this time as I didnt remove the second chamber plug) Looking at the mazda graph for compensating for different rpm, if I extend the graph to accomadate my 150rpm 60psi doesnt seem so bad. I also tested the coils; results- a-b=1.5kohms c-b=open circuit c-a=open circuit results were the same for each coil. No burning on any of the coils. Am I not understanding the procedure for testing or are all my coils faulty on the c-b continuity test? any thoughts on these results? I expect the starter is the original but am not keen on replacing it until I can establish the condition of the engine.

cwumbs

I've just bump started it again and drove down the road. Engine is quite erratic, slow to increase revs, it misfires and probably drops off one chamber completley- intermittently. oil pressure was about 60% of the gauges full travel. There is quite a bad "rattle" which I think is coming from the cat. I then did a compression test whilst the engine was warm--60psi on all three (should be 98psi minimum at 250rpm)) But cranking speed was only 150rpm (probably lower this time as I didnt remove the second chamber plug) Looking at the mazda graph for compensating for different rpm, if I extend the graph to accomadate my 150rpm 60psi doesnt seem so bad. I also tested the coils; results- a-b=1.5kohms c-b=open circuit c-a=open circuit results were the same for each coil. No burning on any of the coils. Am I not understanding the procedure for testing or are all my coils faulty on the c-b continuity test? any thoughts on these results? I expect the starter is the original but am not keen on replacing it until I can establish the condition of the engine.

RIWWP

iTrader: (2)

I ohms tested 3 of my sets of coils in december, 1 set being brand new. I had an ohms reading between 600 and 800 for all 12 coils, all 3 test combinations. The old sets were 750+, the brand new set was high 600s.

A-B, B-C, and A-C, nothing was open circuit

Vyndictive

iTrader: (28)

How are you doing the compression tests? What hardware/software do you have?

nycgps

iTrader: (1)

Im not sure how cold is "cold"

but if its just room temp like say, around 70s. Those numbers simply means FAILED.

The readings between the 2 rotor is too significant. even when cold.

cwumbs

The way I read the mazda manual was that a(+)-b(-) should have continuity with some resistance, c(+)-b(-) should have continuity with some resistance, c(+)-a(-) should be open circuit. Important which way round terminals are!

cwumbs

ambient temp was 50deg f. cranking speed puts it off the scale. The manual says that the standard difference between chambers is 21.8psi and 14.5psi between rotors.
I just need to know if I can extend the manual graph to acommadate my cranking speed. it does look like the pressure requirement does drop off a lot as it approaches 200rpm so maybe my problem is the starter and the engine is fine??