Higgs Boson

you now have 4 documented people (one who did it twice) that broke the LT4 pump with a lash cap. guess what happens to the lash cap when the pump breaks. it doesn't end up back on your workbench, that I can tell you.....

C7&7

Agreed^...I am giving you real world experience of pumps completely busting off of the pump housing that screw to block. 2000 issue free miles without the lashcap.

robert miller

Ok, then use the lashcap & the lt4 pump tell us after a few months how it is going on the motor. It's not rocket science, if people is just trying to help you also.

JHEBERT

I'm not saying to use the lash cap if they are failing, I am saying I don't think there is any benefit to having the larger fuel lobe without using the cap.....

All of my parts are here except the lash cap, I just need to get ahead with other things in my life this week so I have time to start the build.

John

C7&7

Well I can assure you rail pressure is higher with modified cam lobe than it is with a stock cam lobe. I've seen it. Ask any shop.

JHEBERT

Not that I doubt it, but if one where to just "add" .050 to a factory cam lobe, it would not fit through the cam bearing. The installed height of the HPFP also appears to be critical, so if aftermarket valley covers are used it needs to be checked for sure.

I do not want a failed pump, a lash cap loose in the motor, or any of the other headaches involved with said disaster. I am pointing out how the cam manufacturers get the additional lift for the LT fuel lobe. I do, however, want to make the most of the fuel lobe. I feel that the final installed height of the pump relative to the base circle will be key. Obviously, if we are bottoming the pump out, it will fail. If we run a larger lobe with the smaller base circle and no lash cap, do we have enough preload on the lifter...or too much lash?

Thoughts?

LSX Camaro

So getting back to why the pistons broke. I think ring gap was the cause.
I believe the rings caught the cylinder walls and broke the lower ring lands off the pistons. There are clearly no signs of detonation.

FYREANT

I am running 10 pounds on a stock block, and I'm about to turn it up a few more PSI. If the piston broke like it did, its not the ring gap to blame especially at only 8 PSI, it would be the tune that was the culprit.. just my .02..

joemosfet

Looks like your rings were a bit tight. I've seen folks, on the hemi side, file a little off the end of their rings so that when they expand under high boost, they don't buckle and break the piston, which looks like what happened here.

I think the real question is why there was so much heat in the piston that it caused the rings to expand to the point where their ends touched and broke?

robert miller

From the looks of the piston I will agree with both of these marks here. Tune and other on this but not filing down the ring but from it being so tight in these cars. But with the blower on the car it was building up to much pressure in the lower crank case. That with the heat it has to go someplace and these pistons and rings well the weak point will give. How well are good is your catch can in the car.

Now again if your tune is 100% spot on at that low of boost # it had to a
have a few weak pistons and ring sets in the car, and a ton of build up lower crank case pressure that had to go are get out of the lower bottom end.

One more thing here these block are known to be egg shaped down thur the walls by 5,k plus in them look at this on your end also. Robert

FYREANT

I need to eat a little bit of crow here.. I read a cam vendors post that confirmed what you explained about the base of circle being smaller, which took me by surprise. I've had to deal with a lost lash cap in the motor previously, so I am VERY leary of using one, but a very well respected vendor also said that the lash cap was required with a 32% lobe in order to get preload on the lifter. I advised it would be using all LT4 hardware and they advised the lash cap would still be necessary. Though with all the living proof of people busting LT4 pumps with a cap it sounds like the only way to properly address this is to measure the stroke of the pump with the lash cap at top and bottom of the fuel lobe points.

Ant

irun4cops

i realize this thread is somewhat dead, but I ironically keep causing my STOCK lt4 pump to fail on my STOCK engine, in my stock z06 auto.

Car is under warranty.

First one failed at 3800 miles, the second one just failed at 5100. One installed at the factory obviously, the second at a reputable dealer.

Car is an auto, has not been tuned or rev limit changed, stock cam, everything stock.

I don't even play with the paddle shifters. I was in touring mode both times, and pumps failed at 135 which is about 5300 rpm, and the pump failed at 145 or 5600 rpm the second time.

Curious if there is another variable everyone is overlooking here...

Since i obviously have no lash cap or modified cam lobe etc.

It is possible that if the in tank pump didnt send enough fuel to the HP pump, that the HP pump running "dry" would cause it to fail... somewhat like a piston in the engine running lean?

Im really trying to wrap my head around what is causing these pumps to fail. I have a feeling I'm going to be able to lemon law this car before the first year is even over with.

https://www.corvetteforum.com/forums...post1596400345

FYREANT

If it was related to the low side pump not providing enough fuel you would throw a code, I believe a P2635 for fuel pump performance. Have they advised exactly what part of the pump is failing? The shaft? Assembly bolts? Etc. next thing I would have them replace with the pump would be the lifter (between the pump and cam).

irun4cops

not yet, and no they did not on the first failure

irun4cops

They say they are just going to replace the high pressure pump again. I asked them what failed on the pump itself, they said they would get back to me.

The odd thing is, there are racers who run the cars at higher rpm consistently, and have no issues.

Does anyone here know if the actual FLOW of fuel has an adverse affect on the pump? Meaning, at high rpm, the car may still not be at WOT, however, i do achieve WOT for sustained long distances, but oddly, I would think that ALLOWING THE FUEL TO FLOW would be easier on a pump than the opposite. I have never examined how one of these pumps actually work by holding it in my hands in person.

FYREANT

The pump like the motor is warrantied for a reason. You're not abusing the car, you're driving it. That simple. The pump was built to withstand sustained usage as you describe. No reason it should be failing.

Ant

irun4cops

such a puzzle

FYREANT

How long are your "sustained runs"? Other people use their cars on the track and push them hard too without the pump failing. My point is that I don't believe -you- are the cause of the failure.

C7&7

If the car is truly bone stock and the pumps are failing then what you're seeing is just bad luck - period.

I have broken 1 stock LT1 pump and 2 LT4 pumps, but my car was modified and we finally got it all figured out.

My money is on you broke one, and then the dealer just slapped another one on and the replacement pump was bottomed out.

How many miles did the replacement last? How did you know the pump was broken? Did it make a terrible rattle noise and the car run like ****? Have they said what was damaged? Did the pump sheer itself off the housing? If it sheered itself off the housing then it wasn't installed properly.

irun4cops

such a puzzle