Went to buy new spark plugs today in an attempt to raise my gas mileage. Over the past year the car has increasingly become sluggish and feels like I've lost power (aka power band is much more noticeable). Most importantly to me my gas mileage has taken a nose dive. I used to get 32+ on the highway and now I'm lucky to get 25 mpg. Long story short the parts store was slow so the assistant manager and I went poking around today. He noticed, to my dismay, that if you took the oil cap off while idling there was a RIDICULOUS amount of blow by. The PCV valve however pulled vacuum as it should. This blow by from the fill hole sputtered as if coming from a single piston... Engine starts up instantly as it should.

The vehicle hasn't been overheating or losing oil that I know of. It had a bad oil leak that I just replaced about a month ago, since then oil level seems constant. Not sure what to think of it. Exhaust is relatively new as well, and there is a fair amount of soot built up already. I will definitely check my compression, but my tools are locked up at work for the weekend.

I should also note that I've done a lot of work cleaning out the EGR ports (took the fuel rail and access panel off) and disassembled the throttle body to clean what I could. I was careful to put all the hoses back where they came from...but I guess its possible I crossed vacuum lines. Should probably mention that years ago (before the gas mileage plummeted) I took off the resonator and capped the intake and vacuum lines.

Didn't know if anyone was familiar with the vacuum diagrams and pressures of this motor to the degree that they could comment on lines I should check. Right now I'm praying its not the head gasket (I JUST had the timing belt off to replace a leaky cam seal). I'll take any and all opinions/advice. Thanks in advance and hope you all had a safe and enjoyable 4th of July!

 

How did the compression check go?

A lot of what can seem like blowby when you remove the oil filler cap may be nothing more than pressure fluctuations in the crankcase.

Try this experiment; remove oil cap and feel with your hand for blowby. Now take a sheet of paper and lower it toward the filler orifice. As the paper gets nearer the orifice it will be blown up by what you can feel with your hand, but as the paper gets quite close to the orifice it may suddenly 'suck' onto the orifice and be held there by atmospheric pressure.

If blowby were substantial then the PCV would be unlikely to be able to adequately deal with that quantity of gas, and the paper wouldn't get 'sucked' onto the orifice but be blown off no matter how close it was...

If the paper 'sucks' on this would indicate that the net pressure in the crankcase is negative, despite what you may be feeling with your hand or might suspect due to oil droplets being flung out (which have inertia and therefore once moving out won't necessarily go back in easily, and some oil escaping the open orifice with the engine running is normal in any case). This would suggest two things; that the PCV valve is working, and that blowby may well be less than it may appear...

I had a similar concern with my CB7, so much so that I decided to fit an additional breather tube to the cam cover. I fitted the breather spigot and experienced what seemed like a lot of blowby gas at the bare spigot, but when I fitted the tube (about 1 meter long) to the spigot found I had a vaccum at the end of the tube strong enough to hold the tube end onto the end of my finger. To confirm this I can say the tube is clear plastic, and there is never any sign of oil either in the tube nor in the catch tank into which it vents.


Modified by johnlear at 6:03 AM 7/6/2008


Modified by johnlear at 6:08 AM 7/6/2008

 

I'll definitely give that a try, but I know what the pressure used to be like and I'm telling you...this isn't normal!

Tools are locked up till work on Monday, but I'll check compression soon as I get home. Is there any way to really differentiate between bad piston rings and a blown head gasket? Bad gasket will usually have low compression in two adjacent cylinders right?

Was talking to a friend who's a motorcycle mechanic, but he was of the opinion that its highly unlikely its a head gasket. He said the compression would either escape into an adjacent cylinder or into the coolant, most likely NOT into the crankcase. He was saying it would be a vacuum leak, rings, or valve seats (unlikely since its a clearance engine and haven't had timing issues).

At this point I'll do the compression check and spray starter fluid all over the freaking place trying to find vacuum leaks. At this point I'm thinking I have a bad gasket on the EGR or throttle body because I never replaced any of them when I had it apart (they were metal if I remember).

 

OK...just checked on my coolant. I always thought that it was overflowing into the reservoir and cooling off, lowering the fluid level a bit. When I squeezed the hose I could hear coolant gurgle and I thought it was fine. Well...by the time I bled it I found out my coolant level was like 1.5 reservoirs low (filled it with water). Replaced the radiator, hoses, and pump like 2 yrs ago so I know its not leaking. Filled it back up and then went for a spin letting it heat up, afterwards the coolant was perfectly greenish clear, not a drop of oil in it.

Tried the paper over the oil drain suggestion. At idle it is definitely blowing out, but at higher RPM it appears to be pulling a little bit of vacuum. Hard to tell.

I also filled my ABS reservoir back up because I periodically got a light when going over bumps and lately it has remained constant. It was on after filling up. Halfway through my drive I shut off the engine and when I started it back up the ABS light went off but then came back on a few minutes later. I think I overfilled the reservoir just a bit, not sure if that would flip the light.

I'll keep a close eye on the fluid level, I drive about 300 miles a week to work so I should know soon if it drops.

 

Blowby would nearly always be worn rings allowing combustion pressure to escape into the crankcase. I suppose it's possible that you could have combustion pressure escaping into the crankcase and nowhere else.

A blown HG might cause a a loss of compression in adjacent cylinders, or it might not.

You need to do a dry and a wet compression test to tell if lost compression is through the rings or through the valve seats (though I can't see how worn valve seats could contribute to blowby). Do a dry CT first, then do a wet CT test by squirting a cuople of CCs of oil into the combustion chamber, turn engine over a few times then do the CT test again. If the compresion comes up significantly then this indicates worn rings. The wet CT works by the oil temporarily improving the ring seal if the rings are shot, or not if the rings are OK (if no change wet or dry then it's the valve seats).

Now that you've topped it up, keep an eye on your coolant level, both in the overflow tank and in the radiator. Loss of colant with no signs of external leakage suggests head gasket problems, and this can be the case whether or not you have oil and water mixing in either the crankcase or the cooling system.

Good luck!

 

Thanks for the input man! I'll get those compression numbers first thing after work...provided the car makes it there and back.

Anyone got any ideas on that ABS light?

 

Checked the compression, they were all like 200-205 psi so its not blow by I guess...

Changed the spark plugs too which were more worn than I expected, but not to the degree to cause such a drop in power and gas mileage. Changing fuel filter this weekend. I'm frustrated and totally out of ideas.