I need help with an engine problem

[Tommy]

I am trying to diagnose a problem with the engine in my Cobra. My goal is to find the root problem without disassembling more than I need to.

Background

The engine in my Cobra includes the following parts:

Block: 1972 351 Windsor bored to 4.040"; deck height 9.503"

Crank: Speedomotive 408 stroker; Hi nodular cast with 4.000 stroke; balanced assembly; Feb 2004. Main studs ARP 154-5403

Pistons: Probe forged dish top; (10.48 comp ratio zero deck height); Total seal Max Moly rings; coated tops and sides

Calculated Compression Ratio: 9.19 (Gasket: 8.41cc. Piston dish + deck height: 34.2 cc. Head: 60 cc.)

Rods: 5140 forged steel; 6.200 inches long; rod ratio 1.55

Crank & rod bearings : Clevite Tri-metal main and rod bearings

Cam shaft : Comp Cams # 35-000-8RF; Grind # FW 3316F/3317F HR110+4

Lifters and retainers: Comp Cams 851-16

Rocker arms: Crane Gold # 86757-16

Heads, valves, guide plates: Edelbrock Victor Jr. #77189 (60 cc). Head bolts ARP 154-3603. Head gaskets Felpro 1011-2.

Intake manifold: Edelbrock Performer

Current situation

While driving a few nights ago, I heard a sound over the roar of the side exhaust that reminded me of an exhaust leak hiss. Simultaneously, the warning light on my Knock Detector illuminated and stayed lit. The Knock Detector is calibrated to flicker on briefly at its most sensitive engine speed (2400 RPM). I slowed to 2,000 RPM but the noise continued and the Knock Detector light continued to flicker on and off. As I had filled up with gas only a few miles before, I suspected the station had sold me a tank full of 87 octane gas from the 93 octane pump. So I dialed back the timing advance using the device I installed for that purpose on my dash. It didn’t help.

The next day I drained most of the tank of gas and refilled with 93 octane gas from a different station. Even after allowing time for the new gas to get to the engine, there was no change. The noise and warning light were still there.

Today I pulled the spark plugs and did a compression test. All the spark plugs were black and coated in carbon. It’s been several years since I drove the car hard at a track. All it has seen is occasional cruising.

I did the compression test with a fully charged battery and the throttle wide open. Cylinders 1-7 all showed between 200 and 210 PSI. Cylinder 8 showed 125 PSI.

I removed the valve cover for cylinders 5-8 and did a cursory inspection. There is nothing obviously broken.

What I need

I need advice in two areas. First, I need to compile a list of likely problems (e.g., blown head gasket, cracked head, cracked cylinder wall, cracked piston, etc.). Second, I need to know what to do next in search of the root cause of the problem without taking more apart than I need to.

[Roger Bray]

Tommy,
Just a wild guess.
Have you checked the torque on the cylinder heads?

[HSC-ZL1]

Tommy,
I think your next test should be a cylinder leak down to see where the compression loss is. This should seperate whether you have an intake/exhaust valve issue or a cylinder/ring issue. Hope its minor.

[Hotfingrs]

Squirt some oil in #8 and see if comp comes up. That would be rings, I would think a blown head gasket would give low comp in two side by side cylinders. All plugs black and sooty could be a stuck float. Check preload on cylinder 8, see if that helps compression. Check cam lift on that cylinder. Just some little things to do before you start tearing things apart...

[Tommy]

All good ideas. Keep 'em coming.

[Tommy]

To check the preload on the lifters, I went to the Crane web site for instructions. Assuming I selected the correct instructions, they imply I will torque the nut mounting the rocker arm until it forces the pushrod to fully compress the lifter and then a little bit more. I take this to mean that the rocker will be tight against the valve stem and the pushrod even when the lifter is on the flat of the cam. BUT, most of the rockers on my engine can be wiggled easily when the cam lobe is not pushing on them. The rocker arm mounting nuts seem to be properly torqued, but the rockers are looser than I would expect. Am I interpreting this wrongly, or should I be looking at adjusting all my preloads?

[LMH]

Take a close look at the valve springs on #8. I had a 240z that had a similar issue years ago and found a broken valve spring.
Larry

[Hotfingrs]

I use the spin the pushrod between thumb and forefinger technique. Cam lobe is on the flat, spin the pushrod between fingers while tightening the rocker arm until you feel the resistance. Then anywhere from 1/4 to 3/4 turn more. 1/4 turn will give more rpms and higher horse, 3/4 will give less rpms and more torque...so they say.

[Tommy]

Originally Posted by Hotfingrs

I use the spin the pushrod between thumb and forefinger technique. Cam lobe is on the flat, spin the pushrod between fingers while tightening the rocker arm until you feel the resistance. Then anywhere from 1/4 to 3/4 turn more. 1/4 turn will give more rpms and higher horse, 3/4 will give less rpms and more torque...so they say.

The Crane instructions say much the same thing, but also mention the use of spacers if 1/4 to 1 turn of tightening does not produce the specified torque (lifter compression). My concern is that the nut is at the specified torque, but the rocker is loose enough to allow the push rod to spin freely. I assume that means I need to play with the spacer to get the preloads correct. Does that sound right?

[olddog]

Because you said you heard a hissing and compression is low on one cylinder. I have to suspect a blown head gasket. As said earlier typically blows between two cylinders, but compression says it did not. It is possible for one cylinder to leak outside the head or into the lifter valley.

Is there any anti-freeze in the oil? Check this before starting engine again. With engine running is there any air bubbles going into radiator?

I never tried this, but perhaps some soapy water around the head gasket and then fire it up might confirm a compression leak from a blown gasket.

I agree with whoever said a leak down test. You can listen for a valve leaking into the intake, or the exhaust, or into crankcase.

[Bill E]

leak down test, will give the answer.

[LMH]

Yep, I was going to say the same thing... Leak-down test. It'll tell you where the loss is.
Larry

[tirod]

If the rockers are loose, the lifters aren't preloaded. Torque isn't a reliable way to tell, it's just an expression of the amount of friction in the threads of the nut - not how much pressure the rocker is pushing down on the pushrod and forcing the lifter body to compress.

If the pushrods can spin, they aren't preloaded, and the cam timing hasn't been optimal.

[Gaz64]

Originally Posted by Tommy

The Crane instructions say much the same thing, but also mention the use of spacers if 1/4 to 1 turn of tightening does not produce the specified torque (lifter compression). My concern is that the nut is at the specified torque, but the rocker is loose enough to allow the push rod to spin freely. I assume that means I need to play with the spacer to get the preloads correct. Does that sound right?

By the sound of it, you have non-adjustable rocker gear.

If the point of zero lash is less than 1/4 turn from the tightened point of the fastener, then the spacer is too thick.

Then you have do some maths.

[Tommy]

Thanks to all. I'll be doing some more testing today and I'll report when I have something more to add.

[CHANMADD]

Thee are different length pushrods available too. Remember to keep the contact patch at the valve stem in th emiddle of the stem. Maybe spacers and rods.

[mreid]

Please take a picture of your valve train (head with valve cover removed). Your info says you have a 410w and I'd be surprised if it uses non-adjustable (shim type) rockers.

[Tommy]

I got a little done this morning before my day took off in another direction. ... My rocker assembly is not adjustable except by shims to raise the rockers. If the rockers are too high, the answer is longer push rods. It turned out that all of the push rods on cylinders 5-8 were loose enough to spin freely. I loosened and then retightened them to get at least a quarter turn after the push rods stopped turning freely. Six of eight push rods seemed right, but two of them are still not sufficiently preloaded. It looks like I need to get a new set of slightly longer push rods.

In my haste to do some testing, I did something stupid. I put a spoonful of oil in cylinder 8 and repeated the compression test. The good news is that the compression jumped right up to the 210 PSI level of the other cylinders. The bad news is that I don't know if it is because I added oil or because I properly preloaded the lifters. If I had done the compression test before adding the oil, I'd know. .... So, tomorrow I plan to button up the engine and run it for thirty seconds or so to burn out any residual oil in cylinder 8. Then I'll repeat the compression test to see whether the oil or the lifter preload made the difference. ... If I'm lucky, I'll discover that the cause of my noise was insufficient valve opening due to normal wear and push rods that were just barely long enough when everything was brand new.

[Gaz64]

I doubt your low compression on cylinder 8 is to do with no preload on your hydraulic lifters.

I would take all of rocker gear off, to check that all of the valve stems are at the same height.

No preload (now a lash clearance, the opposite of what it should have) and low compression points to bent valves.

[vector1]

do a leak down test, not compression test. i would check the cylinder head torque, which means loosening and retightening each one in sequence, still not fixed, put oil in the cylinder and see if the numbers come jump. if that doesn't fix it....

i would suspect carbon on the valve face surfaces keeping the valves from closing, which would cause the pushrod to seem to be too short.....

fix the rich condition.