Good compression but bad leak down results

[Steve Webb]

Hi guys

My car is slightly down on power. I’ve been through everything including compression tests, fuel pressure, variable intake verified, ignition components replaced, car is serviced very frequently etc etc

So whilst speaking to a guy at the local cylinder head shop about valve and port cleaning he mentioned that even though my compression tests on all 6 cylinders were spot on that I should in addition do a leak down test. I just assumed that if my compression was spot on then there’d be no leaks and therefore I should look somewhere else.

So. Is it possible I could have good compression yet have a bad or moderate leak down result. If so I need to get a tester and see this for myself as this would explain my slight loss of power.

Thanks in advance. Steve

Topic

[Its me]

Interesting… I can’t explain why he would say this…. so we’ll have to wait for your results… I only use a leak down as a supplement to a compression test in order to pinpoint the exact problem.

Karl

[Steve Webb]

Yeah I assumed if compression was bang on spec then everything would be good.

Just to add though we are talking about a 15 year old car with 112k miles on.

[Mike]

Maybe that guy is a “belt and suspenders” type of person, operating with an overabundance of caution. From where I sit, I can’t come up with a good reason for performing a leak down test if the the results of the compression test don’t warrant it.

As for your gradual power reduction, the first easy thing I’d check for is a partial blockage of the exhaust, particularly mufflers and catalytic converters.

[EricTheCarGuy]

When diagnosing performance problems, you have to look at the system as a whole. Right now you’re looking at the engine, but don’t forget about the transmission and the brakes. Also not a bad idea to check for exhaust restrictions as suggested. More info here.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

Please keep us updated.

[Steve Webb]

Thanks guys, I’ll look further in to it. I’ll redo the compression tests also just to be sure.

I had the heads off and manifolds off a little while back. Could all the soot in the exhaust manifold be the actual restriction after 15 years of running? There was definitely no fun blockage but thing perhaps the soot in there could restrict the flow?

Thank you

[Donald]

If you think you’ve got blockage in the manifolds go out to a empty road or highway where you can screw around and not get in trouble. And at temp just blast it hard. Foot to the floor back to back to get high heat into the manifolds and work it loose. It is possible in theory for soot to build up. I’ve never seen a case of it but if the vehicle is babied then possibly. It’s sort of like some vehicles I’ve worked on driven by older people who use it to the store and back or bingo or whatever. They never take it out on the highway or really get it to temp or anything which can lead to excess carbon buildup. I’d check for exhaust restrictions. Check for codes if any. Check for fuel quality. Check fluids in general. Check for sticking brakes including parking brakes.

[Steve Webb]

Thanks for that. As I mentioned there is soot in thee that I’ve seen but I couldn’t tell you how thick the layer is. I’ve no idea if this would restrict anything.

I’ll borrow my head mechanics vacuum gauge and I’ll post up the results as soon as I can.

Thanks for all your replies 🙂

[Steve Webb]

Hi guys, hope you’re well.

So, a quick recap in that car is slightly down on power. I’ve done compression tests on all 6 cylinders and they’re bang on spec.

it was quiet at work today so pulled car in and plummed in the vacuum gauge straight from the brake servo (and tee’d it in so it was all still plumbed up)……. got car up to temp and at idle it was holding a ROCK STEADY vacuum of 20inHG. Perfect!!!

I then brought the rpm’s up to 3k and held it there…..again, rock steady vacuum of 23inHG. PERFECT! This confirms there’s zero exhaust restrictions, zero vacuum leaks and the engine is very healthy. really really pleased actually.

I blipped the throttle a few times and all vacuum dropped to 0 and came back to a steady 20inHG at idle. i believe this to also be completely normal?

while car was in and up to temperature i got my infrared thermometer out and got one of the mechanics to jump in and hold it at 2000-2500k rpm. i checked the inlet and outlet temp. of both cats. Now, my understanding is that the outlet should be 100oF hotter than the inlet. however, i got similarly odd readings on both cats….

drivers side cat:
inlet 180oF
outlet 135oF

pass. cat:
inlet 230oF
outlet 180(ish)oF

Would this have any bearing on performance?

Thank you 🙂

[Steve Webb]

Anyone? 🙂

ps, can a worn/wearing auto transmission cause acceleration loss? if so how come? as I have a healthy engine (very healthy in fact) and haveevidence to support that, I’ve got to conclude this is a transmission issue? its a15 year old box and its never had an oil change. will the oil definitely be burned causing slight slip and thus power loss?

[Donald]

Possibility that you have a transmission problem. You may transmission slip and that would result in loss of power.

[Steve Webb]

Thanks Rattman.

The transmission appears to shift ok and there’s no odd clunks or noises. in fact I’d say it’s smooth changes.

Is there a way to test to see if it’s slipping?

[Author]

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