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Discussion Starter · #1 ·
I have a 'weak' miss on our 1988 Graduate - very prominent at idle.

I ran a compression check yesterday and got the following results:

Cylinder 1: 110 psi

Cylinder 2: 105 psi

Cylinder 3: 90 psi

Cylinder 4: 110 psi

On acceleration the car seems to run ok tho.

I am going to pull the valve cover and check clearances as well as closely inspect the #3 cam lobes.

The original cams are still in the motor and it has about 110k miles on the clock. We've owned it since new and I had a valve job done back in 1994.

Any comments would be appreciated - I'd like to get some thoughts before I pull the head.
 

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I have a 'weak' miss on our 1988 Graduate - very prominent at idle.

I ran a compression check yesterday and got the following results:

Cylinder 1: 110 psi

Cylinder 2: 105 psi

Cylinder 3: 90 psi

Cylinder 4: 110 psi

On acceleration the car seems to run ok tho.

I am going to pull the valve cover and check clearances as well as closely inspect the #3 cam lobes.

The original cams are still in the motor and it has about 110k miles on the clock. We've owned it since new and I had a valve job done back in 1994.

Any comments would be appreciated - I'd like to get some thoughts before I pull the head.
I'm probably covering some pretty rudimentary stuff here so please excuse me for asking but I have some questions about how you ran the test:

1. Was the engine at temperature (180* F) when you ran the tests?

2. Did you pull all of the plugs to do the test?

3. Did you spray some oil into each cylinder before you tested it?

4. Was the throttle wide open when you tested each cylinder?

Your results do seem a bit low especially #3 . IIRC you should be seeing 120 psi minimum for each cylinder.

Drew
 

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Discussion Starter · #3 ·
1. Was the engine at temperature (180* F) when you ran the tests? yes

2. Did you pull all of the plugs to do the test? yes

3. Did you spray some oil into each cylinder before you tested it? no

4. Was the throttle wide open when you tested each cylinder? no
 

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then...... throttle wide open and you will be amazed at the difference - this will be the true readings:)

Sorry, but Don't spray oil into the chambers for the test! This you only do if one of the cylinders is down on pressure, this then indicates if the rings may be at fault!

IMO: full healthy compression around the 170/180 lbs mark - good engine 140/150 lbs - around the 100 mark is getting poor........
Most important is that they are all within 5%-max 10% of one another.

Report back and let us know
 

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Discussion Starter · #8 ·
I did another compression check last night with the throttle wide open n got the following results:

Cyl #1: 120

Cyl #2: 125

Cyl #3: 110

Cyl #4: 130

I also replaced the spark plugs with no joy. I also wanted to add that I can hear a distinct pulsing at the tailpipe (during idle) that I can't get to go away by fiddling with the air box bypass screw nor the idle adjust.

I am afraid I've got a cracked exhaust valve - pulling the head is a necessity at this point.
 

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I can hear a distinct pulsing at the tailpipe...
Typical symptom of a weak or non-firing cylinder. It does NOT, by itself, indicate a cracked/burned valve. Did you do the wet compression test? (a few ml of oil in each cylinder) What are the valve clearances?
 

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Definately check valve clearances & if possible do a leak down test. Valve clearances that are way off can affect compression. A leak down test can pin point the source of the problem without removing anything except the spark plugs.
 

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I personally think that if you had a cracked valve, you wouldn't get 110 lbs reading - so I wouldn't jump to conclusions there just yet....
As Papajam says, now put some oil down the plug holes and retest compression (again full throttle)

If you decide to test the valve clearances, personally I would first check that all the cam bearing blocks are properly torqued before you measure (you never know what a PO has been up to!) At least then you will have a correct base value to work with, if you need to remove cams and reshim the tappets.
 

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dirty injectors

I agree with other posters. Check valve clearances first. In my experience, a cracked or burned valve will be tight. But 110 is too good for either. My guess is dirty injectors. A rough idle or misfire that goes away with greater fuel flow often indicates a bad spray pattern.

~Chris
 

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Discussion Starter · #14 ·
Thanks for the additional replies. I am planning on checking clearances and have to locate the new injectors I have - they are somewhere in the garage (I believe I know where)..... I'll post once I get an update (might be a couple of weeks)....
 

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Discussion Starter · #15 ·
I got the car back together and it is running great!

I pulled the valve cover and the #3 exhaust clearance was maybe .002"

I ended up filing down the spacer for that valve down to .039" (smallest ones avail are .051").

I will have to go back in there in 6 months and check again, some of the valves were within spec but on the tight side....

Thanks for all your help! I'll also post updated compression test results soon! Now off to drive it while summer days are still warm!
 
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