Low cranking speed during compression test.

[shiwei]

I did compression test for my 2006, Nissan Sentra Spec-v, 134K miles, 2.5L engine the passed weekend. Several weird phenomenas:

1. During the compression test, I removed all spark plugs and fuel pump fuse, throttle to the floor. But the engine cranked very slow, around 2 cranks per second. I measured the battery, it was 12.6V.
2. After compression test, I reconnected all plugs and fuse, the engine just cranked but wouldn’t start. (I tried to start the car for 6 times, no luck. Had to stop cranking to save battery) I unplugged the spark plugs, and found fluid (oil or gas) at the tip. (I also left the spark plug holes open for 10 mins in case of flooded engine)
3. Two days later, I connected a spark plug tester, the car just started as normal.
4. These days, I found the MPG drops from 28mpg to 26mpg. I always use Shell gas. It also seems the engine power drops a little bit.

I tried to replace the crankshaft position sensor, but gave up since the car works now.

So my questions are:
1. Why the cranking speed is so slow?
2. Why I couldn’t start the engine but could started 2 days later?
3. Why the MPG drops?

Could it be due to the crank shaft position sensor?
Any other reasons?

Thank you, pros.

Topic

[college man]

[quote=”shiwei” post=151103]I did compression test for my 2006, Nissan Sentra Spec-v, 134K miles, 2.5L engine the passed weekend. Several weird phenomenas:

1. During the compression test, I removed all spark plugs and fuel pump fuse, throttle to the floor. But the engine cranked very slow, around 2 cranks per second. I measured the battery, it was 12.6V.
2. After compression test, I reconnected all plugs and fuse, the engine just cranked but wouldn’t start. (I tried to start the car for 6 times, no luck. Had to stop cranking to save battery) I unplugged the spark plugs, and found fluid (oil or gas) at the tip. (I also left the spark plug holes open for 10 mins in case of flooded engine)
3. Two days later, I connected a spark plug tester, the car just started as normal.
4. These days, I found the MPG drops from 28mpg to 26mpg. I always use Shell gas. It also seems the engine power drops a little bit.

I tried to replace the crankshaft position sensor, but gave up since the car works now.

So my questions are:
1. Why the cranking speed is so slow?
2. Why I couldn’t start the engine but could started 2 days later?
3. Why the MPG drops?

Could it be due to the crank shaft position sensor?
Any other reasons?

Thank you, pros.[/quote]

Was 12.6 measured while cranking?
Can’t explain the no start other than sounds electrical. the battery sounds weak or connection problem.
dropping 2 mpg in colder weather will do it. No codes?

http://www.ericthecarguy.com/faq/finding-and-fixing-the-causes-of-poor-mpg

http://www.ericthecarguy.com/faq/solving-automotive-no-start-problems

[Mike]

1) The cranking speed is slower because it requires more power to turn the engine without compression. I don’t fully understand the scientific reason for this, but it is known that an assembled engine being turned at speed by a electric motor (for airflow testing) draws substantially more current to maintain a given RPM if the spark plugs are removed.

2) If the engine was turned for compression testing in a way that allowed fuel to be injected, you could have a no start due to flooding or a lack of compression due to the fuel (which is a solvent) washing the naturally occurring oil residue off the cylinder walls. One newer engines if the oil is washed of the cylinder walls during a flooding condition, oil has to be put in thru the spark plug holes to restore normal compression and allow starting. This has to do mostly with the low-drag piston rings and thin oil that have been used the last few years to improve fuel economy. Older engines (including your 2006) seem to be able to get the walls re-oiled enough to start just from oil sloshing around during cranking, although it usually takes at least several seconds of cranking before it threatens to actually start.

3) There may be some scuffing of the cylinder walls due to lack of lubrication during all the attempted starting, which would likely have a slight effect on ring sealing, which would directly affect performance and fuel economy. Understand that although I’ve dealt with a lot of this kind of issue, I’m being quite speculative at this point.

[Gary]

[quote=”Fopeano” post=151299]1) The cranking speed is slower because it requires more power to turn the engine without compression. I don’t fully understand the scientific reason for this, but it is known that an assembled engine being turned at speed by a electric motor (for airflow testing) draws substantially more current to maintain a given RPM if the spark plugs are removed.

2) If the engine was turned for compression testing in a way that allowed fuel to be injected, you could have a no start due to flooding or a lack of compression due to the fuel (which is a solvent) washing the naturally occurring oil residue off the cylinder walls. One newer engines if the oil is washed of the cylinder walls during a flooding condition, oil has to be put in thru the spark plug holes to restore normal compression and allow starting. This has to do mostly with the low-drag piston rings and thin oil that have been used the last few years to improve fuel economy. Older engines (including your 2006) seem to be able to get the walls re-oiled enough to start just from oil sloshing around during cranking, although it usually takes at least several seconds of cranking before it threatens to actually start.

3) There may be some scuffing of the cylinder walls due to lack of lubrication during all the attempted starting, which would likely have a slight effect on ring sealing, which would directly affect performance and fuel economy. Understand that although I’ve dealt with a lot of this kind of issue, I’m being quite speculative at this point.[/quote]

Number 3 shouldn’t be an issue. Even at a low RPM, the oil pump is stlll rotating. Unless the crank was spun backwards, it was still getting lubrication.

However, if the cylinders were flooded with fuel, that would have diluted the oil to the point of nothing and could cause such scuffing. Just my .02

[Mike]

Thing is, cylinder walls are normally splash lubricated as opposed to being directly lubed by pressurized oil. At starting RPM, there is little oil splash up above the crankshaft. If there is fuel actively leaking down the cylinder walls, the splash at that RPM is really pissing up a rope trying to get the whole wall area coated. The exceptions of course are rifle-drilled connecting rods that squirt some of the rod bearing oil on the thrust side of the walls and piston crown oil squirters, but those are somewhat exotic features I would bet are not on the SE-R Spec V. According to Wikipedia, it’s got the same QR25DE as a Altima 2.5S, which has been a problematic model including the engine.

My speculation didn’t even take into account the possibility of the oil being diluted by fuel and subsequently providing poor lubrication in general.

[Gary]

[quote=”Fopeano” post=151304]Thing is, cylinder walls are normally splash lubricated as opposed to being directly lubed by pressurized oil. At starting RPM, there is little oil splash up above the crankshaft. If there is fuel actively leaking down the cylinder walls, the splash at that RPM is really pissing up a rope trying to get the whole wall area coated. The exceptions of course are rifle-drilled connecting rods that squirt some of the rod bearing oil on the thrust side of the walls and piston crown oil squirters, but those are somewhat exotic features I would bet are not on the SE-R Spec V. According to Wikipedia, it’s got the same QR25DE as a Altima 2.5S, which has been a problematic model including the engine.

My speculation didn’t even take into account the possibility of the oil being diluted by fuel and subsequently providing poor lubrication in general.[/quote]
Eh, your right. I was thinking about bearings which are pressure lubed….not the walls themselves. I’d have to agree with your theory. Also, add my theory in about the oil being diluted by fuel and you have a compound issue. I always disable both fuel and ignition when testing for this reason.

[shiwei]

12.6 was measured when not cranking.
No codes yet.
:cheer:

[shiwei]

Very scientific explanations. Thank you, pro. B)

[Author]

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