Awhile ago I posted about my Dad's 86 that has a steady miss. Thru the help of the forum, we found 3 leaking injectors. We had them cleaned/replaced at Cruzin Performance and they seem to be fine. However, the miss is still here.

The miss is steady but not as severe as when a plug wire is off. We have replaced all of the ignition stuff except the distubuter itself with known good parts from my 90 with no success.

We readjusted the valves to the GM specs with no change. We can't SEE anything wrong with the valve train while the car was running - but that doesn't mean much.

Here is where we are stumped - We hooked up a vacuum gauge and it steadily bounces from about 8 - 12 in at idle. What would be a likely cause for this? The gauge reads a steady 17in on the 90 (ie the gauge is OK).

We have the parts to make a leakdown tester here but, thanks to LPE, we have to pull off the headers before we can get anywhere near the plug holes to run the test - :cuss

Anybody have any ideas what to do about the wacky vacuum readings? Intake leak? Head Gasket? Valve problems?

Dale

 

Clean Throttle Body....leak at the Throttle Boddy?

 

Fluttering vacuum can be a sign of a bad valvetrain component. Do a compression check and post your results.

 

Chevy Guy has good advice. Wait until the engine is cold, remove plugs and run and record the compression on each cylinder. My money is on one or more cylinders with low compression.

 

Unless you have a great big cam in that thing, you should be pulling a lot more vacuum. I agree with the above post on the compresson testing.

 

Those are the classic characteristics of a vacuum leak.

The leak can be from:

Hose or hose bibs, including the hard plastic ones,

Underside of TB, power brake booster, PCV at intake, EGR, A.I.R., etc

Intake manifold to head sealing surface,

Injector "O" ring where the injector enters the intake manifold,

Fuel pressure regulator,

Hoses to the charcoal cannister/cruise control,

Cracked power brake booster (I just walked that road)

Plenum to runners and runners to intake manifold gaskets
REMEMBER, the runner to intake manifold/plenum are side specific - Are you sure you installed the correct gasket on the correct side?

Did you remember to install the two hidden runner to intake manifold bolts?

You can use water or spray carb cleaner or even propane. Water is safer; carb cleaner allows better precision.

You can get a few cans of spray carb cleaner. Use the thin plastic tube that comes with each can so you can precisely direct the spray. Let the engine idle and begin spraying. When the idle changes you've found the leak.

There's a hose bib on the underside of the TB (driver's side) and two more on the passenger side of the plenum just about the fuel pressure relief Schrader valve.

With enough time and patience, you'll eventually find it. Took me a while to discover I'd installed two "O" rings on one injector, which caused a vacuum leak. One "O" ring had gotten stuck in the intake when I removed the stock injectors. So when I installed the new injectors, the #3 injector ended up with two "O" rings.

Could be a bad plug wire or a spark plug with a hair-line crack (had to see unless you look really closely). Let the engine idle in a completely darkened area and look for arcing. Attach a timing light to each wire, in turn, and watch for similar flashing; if one flashes intermttently (or different) from the others, that's where I'd concentrate my effort.

Loose rotor (plastic screws are the worse offenders) or with a crack or burn through hole; arcing within the distributor cap (you should be able to see carbon tracking).

Since you had the injector work done by a shop, I'd first ASSUME they did the job correctly BEFORE I began tearing into the installation of the runners, etc.

Hope this helps. Good luck.

Jake


[Modified by JAKE, 4:18 PM 3/9/2004]

 

Well, after dropping the entire exhaust system (!@#$ing LPE headers) we got the results.

1: 186
2: 192
3: 192
4: 190
5: 204
6: 186
7: 204
8: 0

I'm no master mechanic but I think #8 has a problem. We are going to try to blow air into the cylinder to see where the leak is (piston, intake or exhaust).

I will post results in a bit...

 

Problem found!

I'm not sure how we missed it the first time but the #8 exhaust spring is broken.

Question: How do I figure out what kind of spring to replace it with? Nothing in the literature we got from the previous owner mentions valve spring P/Ns or specs.

Thanks All!

Dale

 

Two ways to tackle the problem:

Replace all the springs, which is what I'd do, or

Remove one of the other springs, take it to a machine shop and have them check the diameter and pressures.

I can't see how a broken exhaust valve spring can give you a ZERO reading on a compression test though unless something else is wrong too. Since the intake valve is still opening normally, (presumably) you should get a gauge reading UNLESS the intake and/or exhaust valve is bent.

If one of them (or both) are bent and not seating, you should be getting back flow through the intake manifold if it's the intake and out the exhaust port if it's the exhaust valve that's the offender.

As the #8 rises on the compression stroke, the mixture is forced back through the leaking valve(s).

Keep us posted.

Jake

 

You sure? :D :lol:

Seriously though did you do it a few times just to be positive you had the cam in the right position? :)

 

We called LPE and they are going to ship us new springs for $50. This is a complete set so we will fix them all (Oh Goodie!).

After we replace the springs we'll run a leak down to see if everytiing looks OK before we put everything back.


I'm not sure I follow ya :confused: All we did so far was a cranking compression test. And we did do it a few times because we were a little surprised by the 0 number. I still have to build the leak down tester but we will use it before we are done.

- Dale

 

I just mean are you POSITIVE that both valves were closed (or at least supposed to be) when you did the test on #8.

 

So, if you still have to build a leak down tester, that means the previous tests you ran were cranking compression tests, right?

Did you hear any compression leaking during the cranking? Leaking out of the exhaust port, intake port spark plug hole or into the oil pan?

If you got a Zero reading there has got to be some serious cylinder leakage somewhere, unless the intake valve isn't opening.

Jake

 

Correct. All I have done so far is a cranking compression test hence the reason I was confused by scorp's post. A cranking test doesn't involve me setting cam position, does it?

I didn't hear anything during the cranking so I removed both rocker arms from #8 so the valves are as closed as the springs can make them. I then pressurized the cylinder to 90psi or so and could feel air escaping thru the exhaust port.

I am guessing that it is possible that the valve could have been schmucked by the piston if things are tight enough in there. This should show up in a leak test after we repair the spring right? Or would a bent valve be the likely reason for the leak now? Can a simple broken spring cause a bent valve?

- Dale

 

[SNIP]

Correct. All I have done so far is a cranking compression test hence the reason I was confused by scorp's post. A cranking test doesn't involve me setting cam position, does it?

No it doesn't. Just make sure there is a good seal at the spark plug hole.

Did you hear any compression leaking during the cranking? Leaking out of the exhaust port, intake port spark plug hole or into the oil pan?

I then pressurized the cylinder to 90psi or so and could feel air escaping thru the exhaust port.

Sounds like a bent exhaust valve. Remember, the piston is always chasing the exhaust valve, so if the exhaust valve spring breaks, the exhaust valve cannot close in time to keep the piston from catching up with it.

I am guessing that it is possible that the valve could have been schmucked by the piston if things are tight enough in there. This should show up in a leak test after we repair the spring right? Or would a bent valve be the likely reason for the leak now? Can a simple broken spring cause a bent valve?

Yes, see above.

- Dale[/SNIP]


[Modified by JAKE, 1:00 PM 3/11/2004]

 

Disreguard that, I got cranking and leakdown twisted somewhere.

 

Update:

We replaced the spring on #8E and it still fails the pressure test. I guess this means the valve it bent and head comes off. O happy day.

We started pulling the SuperRam off and when we pulled the plenum lid we found a ton of condensation (mostly water but I can smell other stuff in it) over the even bank runners. I am guessing this is caused by the exhaust valve being open. Any other likely causes?

Does anyone have any tips for removing/installing the head easily? I am all ears.

Thanks

Dale

 

:lurk:

 

Yep, set aside the best part of a day.

I had mine off a couple of weeks ago.

Your setup may be slightly different than mine, but here's what I did.

Jacked the car (stands of course) and dropped the starter and unbolted the gutted cat from the rear "Y" pipe. I unbolted the headers and they were able to drop just enough to allow me to get to the lower row of head bolts on both heads.

I didn't even have to remove the headers from under the car, just left them dangling.

I had to unbolt the passenger side cross brace by removing the outboard bolt and loosening the inboard bolt.

The rest is the usual stuff, remove the three braces; one to the alternator, one to the A.I.R. pump and one to the A/C.

Removed the two nuts near the water pump that hold the A/C bracket and the hidden torx head bolt. Removed the belt tensioner, electrical connectors at the front of the intake, distributor, runners, plenum, fuel rails, intake manifold, air filter spark plugs, and MAF, etc.

I removed the coolant senders - one in each head - to allow coolant to drain and also removed the lower rear-most head bolt first to allow more coolant to drain.

Wasn't too bad, but it'll go faster with someone to help out.

Jake

 

Thanks Jake!

We got the head off with out too much trouble.

We pulled the #8E valve out and found it has 0.003" runout at the head of the valve. Using the same method, #2E was the same. We measured this by putting each valve in our drill press and placing a dial indicator against the same spot on each valve and turning.

Seems that the valve is OK. It was pretty carboned up - maybe a little more than #2E but not much. Could the little bit of carbon cause the leaking problem?

Any ideas?

Thanks

Dale