Hello.

I sold my corvette back in the spring. Kid has recently had a problem with it so I agreed to look at it.

Car is missing, and I can't figure out why and making noises.


First - the car is getting knock counts, like tens of thousands so it's pulling back timing to the tune of 12+ degrees under throttle. Does NOT get knock off throttle.

Next - the rockers make noise NOT at idle, but under throttle. These are aluminum 1.5 roller rockers. I assumed out of adjustment, so I re-adjusted them. They are dead quiet at idle, but if you snap the throttle they all tap. This is a hydraulic roller cam. I removed the serpentine belt and verrified - the noise is coming from the rockers. I'm *assuming* this is causing knock counts listed above.

Finally - I disabled the knock attributes in the prom, and took the car for a test drive. Fuel, spark all looked normal. Vac HOLDS FINE under throttle and cruise, cruises around 40kpa, idles at 50kpa. But the car runs rough, and misses. I didn't see oscillations in the vac gauge, so I don't think it's a bent valve or anything. But something is causing it to run rough.

The current owner did plug wires, plugs, cap rotor thinking it was electrical. I did an injector test (pull each one at a time and see if it skips - all injectors seem to work, at least, somewhat because the motor will skip if you pull any one at a time).


I'm at a loss. This isn't my problem, but I'd really like to help the kid out. But I can't imagine what would do this.

Assuming they are related, what would cause a tap only under throttle, AND cause a miss/rough running ? (yet not screw up vac in an obvious fashion).

One last piece of info. The kid did a compression test before bringing it over. He said most cyls had 170psi, however 3 cyls on the right bank (in a row) had around 145psi. I'm curious if that is enough to cause the 'miss' and poor performance, and the rocker/valve/lifter noise is an unrelated issue. Whatever the rocker issue is it ONLY happens under throttle, not idle and the knock counts in the ECM follow it religiously!


-- Joe

 

I have to be honest with you i find it extremely odd that a missfire didnt show up in the vacuum reading although i will say your vacuum reading is a bit low assuming you don't have a fairly large cam. it should be around 20 hg and a reading of 50 kpa is roughly 14.7 hg. Now a possible cause for a low reading among many other things is the ignition timing being too retarded which if the ECM was pulling back the timing makes sense. This late ignition timing would account for poor performance. The compression test results look fine as far as according to what factory specs says is an acceptable variation between cylinders and should cause no performances issues that would be noticeable. Although it is odd that there all roughly 170 and 3 in a row are 145 especially if the readings were all exactly the same. If they were the same virtually exactly that to me points to a head gasket issue but that usually only happens between 2 cylinders not 3. You may want to consider doing a leak down test as its really better than just a standard compression test. However the big question i want to ask is how do you know its a valve noise at all? Valve noises, although may intensify with RPM, dont usually go away at idle especially if theres an issue bad enough to make the car run rough. However something like the bottom end could do that. When you give it more throttle and put it under a load especially there more pressure being developed in the cylinder which will equate to more noise coming from the bottom end if theres a problem with too much clearance between a rod and the crank. This could trigger a knock and cause some running issues like the miss. A simple test for this is pull one spark plug wire at a time and take it for a drive you you notice pulling one A. makes the car run no worse you know you've found the cylinder at fault and B. if the "valve" noise decreases you know its not a valve noise at all its actually a bottom end issue.

 

Ah.. Yeah I failed to outline the combo. It's a 10:1 motor, forged bottom end, h beam rods, crane cam (222/230, on 112lsa 110 ica). Has a small powerdyne blower, about 6psi.


I have the bin set to run 24 degrees at idle. The knock code was pulling timing under throttle, but I disabled that for the test drive so there was no retard going on. But it still misses. I think I have it set to ramp out to about 28 degrees by 3k. If it didn't have the blower I'd ramp it out to about 34-36.


One thing I considered, head gasket. I suggested he do a leak down test this week.

There is a good chance, the combo of high compression and the small blower is just too much for the felpro 1010 gasket. I had recommended a few times he sell the blower, and run naturally aspirated.

Took the serpentine belt off, took the valve covers off, and listened to it.
they all clatter at RPM. It's like if you are adjusting valves with motor running. You crank them down a half turn to set the preload and they go quite. Then you give it throttle and they make noise again.

The noise is in series with the rocker motion, you can almost hear each individually.

Normally a bottom end noise doesn't do it on a throttle snap on a parked vehicle. Plus, I can't imagine that happening. The bottom end should take 700+ hp all day, and it has less than 5k on it. Clearances are dead nuts on for the type of oil used (30 weight), and the oil pressure is perfect (50 or so psi at idle, higher with rpm).


The thing is, it's not 1 knocking noise, it's like a machine gun. It sounds like a good bulk of valves. Not just one rod or something.


Thanks for the response!

-- Joe

 

Interesting. Idk i still think the vacuum is a bit low. I mean thats a pretty good cam assuming those specs are at .050 but ide still expect more vacuum than that. Granted though its hard to say especially since its FI thats not exactly my specialty lol. My next question would be what kind of lifters are you running just factory stuff or aftermarket? Lastly have you by chance removed a rocker and inspected it for damage or wear? Maybe even temporarily swap them for standard stamped steel rockers? I dont know the lift specs but im assuming that the clearances were checked to ensure no coil bind ect?

 

Hello,

Yeah comp camps roller lifters. No factory junk. I did not pull a rocker, but if he does a leakdown test this week I will swing by his house and look at them.

The springs are good for .540 lift (trickflow heads, trickflow springs for hyd roller cam). The cam is .528 lift, so that is plenty of room for coil bind.


However, I cannot guarantee a spring did not break. I had programmed a rev limit in the bin for around 6,000 rpm, but if he accidentally downshifted from 4h to 1st he could have spun the motor to 7k and floated the valves.

Swapping to standard rockers is a great idea. Should probably also check the springs when he does a leakdown test.


Thanks!

-- Joe

 

Anyone else?

-- Joe

 

He took the intake off after pulling the left side valve cover.

#3 pushrod was bent 90 degrees, and the lifter top had come apart/popped up. (oe style comp cams roller hydraulic lifters).


Not sure what happened. I asked him if he over-revved it, he said NO. I had a rev limiter set in the computer, so there is no way he could have WOT over revved it, however he could have stuffed it from 6th into 1st or something like that but he says no.

I guess he's replacing a few lifters, pushrods, and putting it back together. I recommended he remove the blower and leave it naturally aspirated (10;1 compression) because I can't rule out that it won't detonate in hot weather.

-- Joe