Loss of pressure (vacuum & compression test results

ertaisi · Post by **ertaisi** » February 25th, 2005, 12:58 am

Is it normal for there to be a hissing sound coming from the intake manifold (or very near it). I heard the hissing and it was obviously coming from the intake system somewhere. When I disconnected the airbox and hose to the TB, the car obviously wouldn't start, but the few turns it got still made the hissing sound somewhere past the TB.

Also, while messing with all that, I noticed when I rev my engine air comes out of the intake. Shouldn't it be sucking more air in? I wouldn't think it should ever blow air out...

What · Post by **What** » February 25th, 2005, 5:53 pm

check for leaky gaskets/cracked hoses/ etc.

If you slam the throttle shut or even let off it should send a pressure wave back

JWMotorsports · Post by **JWMotorsports** » February 25th, 2005, 9:06 pm

Hook up a vac gauge to the manifold and see what it does. It should stay steady and above 15 in/hg at idle if all is normal. Also run a compression test... all cyl should show at least 130psi and vary no more than 5-10 psi between the highest and lowest tested cyl. These two tests will help narrow down the problem. You can also check for leaky vac lines etc... by spraying them with carb cleaner, when it hits the leak you'll hear the rpms rise a little for a sec. Please let us know your results and we can start tracking down the problem. Also make sure no vac hoses or caps have become disconnected any where (like on the manifold).

ertaisi · Post by **ertaisi** » February 26th, 2005, 6:53 am

I've never heard of a vac guage, but I'll look around and see if I can find one. I'll have to get the compression test done at a shop, maybe I'll have them do the vacuum test too. Thanks.

To elaborate on the intake blowing air...I understand there would be a backpressure wave when the throttle closes, but it's blowing air constantly when I hold the throttle open.

JWMotorsports · Post by **JWMotorsports** » February 26th, 2005, 2:10 pm

Is the air hot or cold? Any smoke? What does it smell like?

The vacuum & compresion test gauges can both be bought at a auto parts store for what a shop will charge you to test it. These gauges come in handy, I'd recommend buying them. They are around $20 each. To test compression you'll need to disconnect the fuel pump so the engine won't start (the harness is under the back seat, easy to get to). Also disconnect the ingnition coil harness. Remove the spark plug and screw the compression gauge in. Have some one crank the motor until the needle stops rising (should only take a couple turns). Read the gauge and record the cyl No. and psi it made. Do the same for the other 3 cyl. Cyl. No. 1 is on the passenger side by the timing belt and cam pulleys. Let us know your results.

Domnknpimp · Post by **Domnknpimp** » February 26th, 2005, 11:38 pm

is it a hissing sound or more like a whistle? I had a problem w/ mine where it would whistle really loud and high pitched when ever i was idling... turned out to be the TB gasket

ertaisi · Post by **ertaisi** » February 27th, 2005, 4:08 am

JWM, thanks so much, I had assumed the tests required expensive machines, heh.

The air is cold. No smoke. Didn't smell it, but I had my ear cupped on the intake and didn't notice smelling anything so probably not a strong smell if any.

Dom, it's a distinct hissing sound. I'd imagine one gasket leak could sound like whistling while another sounds like hissing.

ertaisi · Post by **ertaisi** » February 27th, 2005, 10:27 pm

I picked up a vacuum gauge, but the parts store was sold out of compression gauges. I'll have to pick up one tomorrow elsewhere. I hooked the gauge up to a line coming off the top of the manifold about an inch after the throttle then to another line on the far left end of the manifold (pretty sure it came off the top, if not it was back) with same results. All results have been adjusted for 1000 ft. above sea level.

Idle: 20HG, minute slow oscillation of needle of about .2HG with a slow dip every 5 seconds or so of .5HG which occured in tandem with a dip in idle RPM and engine sound. At idle, the needle vibrated and I thought it was normal but it holds solid at higher RPMs so i'm not sure if it means something or not.

2000 RPM: 22HG steady

3000 RPM: 22HG steady

Quick stab of throttle: dropped down to a reading of 0, rebounded to 24-26HG, then settled to idle a couple seconds later

I did a bit of research on how to interpret the readings at autorepair.about.com and it appears my readings are normal if I understand everything correctly.

Compression test tomorrow, will have results midday or evening.

ertaisi · Post by **ertaisi** » February 28th, 2005, 12:11 pm

Compression test results are in and although they're comforting, they're frustrating at the same time.

Cylinder 1: 170psi
Cylinder 2: 185psi
Cylinder 3: 195psi
Cylinder 4: 190psi

Online manual says 180psi max, so I'm guessing carbon buildup is my culprit in that area...but I don't think that could be causing my poor performance.

I knew when I bought the car it had a valve cover gasket leak, and I noticed the seats of my plugs are coated with oil, so I'm going to try to find a replacement and get it in tonight as well as putting in new plugs although they are dry. Looking at the top of the 2 and 4 pistons, they appeared oily. I don't know if that could be from the valve cover gasket or not. If it is, it's only fair to assume my plugs were dry when I took them out this time but they aren't all the time.

Well, I'm comforted to know I don't have a blown head gasket or bad piston rings, those would be a PITA and/or expensive...but I'm still frustrated, I'm at a loss as to what my problem is.

What to do next? Ignition system? Fuel?

jimishadow · Post by **jimishadow** » February 28th, 2005, 1:08 pm

chec comp again if cyl 1 is same id say you found your problem

ertaisi · Post by **ertaisi** » February 28th, 2005, 2:47 pm

According to the online manual, minimum psi is 142 and maximum is 185 with a difference of no more than 28psi. 170 is 25 away from my max reading of 195, so it's close but not over. What're you basing your conclusion off of?

jimishadow · Post by **jimishadow** » February 28th, 2005, 3:16 pm

what is the actual problem?is it the noise or is there a lac of performance?how many miles are on engine?where does temp gauge sit when warmed up?

ertaisi · Post by **ertaisi** » February 28th, 2005, 3:31 pm

Well, I went in search of a new valve cover gasket and ended up special ordering one from the Mazda dealership. While I was at the parts store, though, I decided to pick up a new set of plugs for the heck of it.

Got home, went to gap them but they were pre-gapped for me. Out of curiosity, I checked the gap on my old plugs...they were .15 over spec! Somehow, between the time I bought my car and it ran fine and a week and a half later when the engine problem started, they'd either magically increased their gap or the engine just realized it didn't like 'em. After putting the new plugs in and warming her up, I ran up the street to grab some smokes. I couldn't make my engine miss for the life of me.

I'll see in the next few days if the problem comes back before I get the gasket in, but I'm hopeful I'm good for now. We'll see after I get her real warmed up where the problem was most prevalent.

jimishadow · Post by **jimishadow** » February 28th, 2005, 3:50 pm

engines dont dislike parts,if the plugs are new or old and there is a problem they can foul up in a few minets,answer the other stuff i asked ya and describe exactly what your car does.i fix mx3 s almost every day,i have sold 10 i fixed up with 4 cyl and 3 gs.im working on one today pulling motor and tranny to put in same from low miles wreck.also 4cyl,all the cars i sold run great.these are good quality jap hot rods so if ya tell me the answers to what i ask ya we can get your car running rite.i have lots of parts also from 3 wrecs i have how many miles on engine???

ertaisi · Post by **ertaisi** » February 28th, 2005, 3:57 pm

jimishadow wrote:what is the actual problem?is it the noise or is there a lac of performance?how many miles are on engine?where does temp gauge sit when warmed up?

Temp gauge sits smack dab in the middle when it's warm. Just under 95k on the engine. I was thinking the noise might've been a symptom of the performance.

My problem was this: When the problem began, I'd get a punch of power at varying RPMs depending on throttle position. Wider open the throttle, later and harder the punch occured. The condition degraded to my engine blatting and missing at me whenever I tried to accelerate normally, so I had to coax it up to higher RPMs where it would perform alright unless I tried punching it.

My guess is that the posts on the plugs were gradually eaten away from all the missing going on, and my untrained eye thought they looked ok the first time I pulled them without checking gap.