skinny z

Has anyone ever had the mechanical fuel pump cam eccentric go flat on them?
Looks like that's the case regarding my 1/2 psi fuel pressure (and getting worse).
I guess it's the same reason my old hydraulic flat tappet went away years ago. I never would have thought it would happen to something as benign as the pump lobe.
Electric pump here I come.

Damon

No, can't say I've had that problem. I've made the "brass tipped fuel pump pushrod" mistake, though! What a nightmare!

The load is just a tiny fraction of that applied to the lifters. And, of course, there is no "lobe" with a pointy nose to wear off of it- just a big offset circle. Overall, it leads a very easy life compared to a lifter lobe.

I'd yank the fuel pump pushrod and see how it looks on the end. If it wiped the fuel pump eccentric off the cam it should have obvious damage as well.

Also, I've "tested" the fuel pump eccentric before by holding the fuel pump pushrod against it with my finger and cranked the engine over with the starter. It should move about 1/2". If it's "flat" or the cam never had a fuel pump eccentric to begin with it will be obvious.

Given that you have an aftermarket cam (which will almost always have a fuel pump eccentric, unlike some later factory cams) and the exceedingly unlikely chance of it going flat..... I have a little part of my brain telling me your fuel pressure problem may lie elsewhere.

Apeiron

I'd look for some kind of lubrication problem, or a jammed pump.

skinny z

The problem has progressed from an initial lack of fuel at top rpm. Last week I couldn't amke it to the 1/8 mile stripe and today it wouldn't even run. Fill up the bowlks manually and you get a few minutes of action then dry. It used to be able to supply 5 psi and has dropped past 3 psi at idle and now nothing.

An earlier test showed maximum push rod travel at slightly more than an 1/8th". (Less than spec which is slightly more .400")

I also use a Moroso light weight push rod. Examination showed no undue signs of wear.

This is the 2nd new Edelbrock 1721 pump and the result has steadily gotten worse. With a short length of suction line and an open can of fuel, the pump will draw nothing. If I elevate the fuel can to above the pump, 1 - 2 psi is maintained. Nothing more even at higher rpms. All of fuel system is top notch and new this season so I can't see the problem being elsewhere. Especially after I've eliminated everything from the tank to the pump with that test. Testing the pump by hand (cranking the lever manually) shows good suction.
My thinking is that the same lack of engine oil additives that has killed many flat tappet cams has claimed my eccentric lobe too. Keep in mind that this XR276HR cam has had a hard life and a lot of miles. In excess of 60 000 I'd say. Many days of 6 000 rpms. This same setup has gone 12's (at 108mph) as while providing excellent fuel economy for several years. Same cam, fuel system (from fuel tank and pickup to carb).

Tomorrow is a Holley Blue pump and external regulator installation day. I'll post up after.

sofakingdom

iTrader: (1)

I'd be looking at the suspected failed parts before just throwing money at it that way.

skinny z

The mechanics behind a mechanical fuel pump are relatively simple. Lubrication is by way of drain back and splash from the lifter valley. Cam eccentric, push rod and pump. That's about it unless I'm missing something. Keep in mind the ENTIRE system was part of a successful and long lived build (up until this started to happen).
I've dissassembled the latest pump (both 1721's where new out of the box). I saw nothing unusual with the diaphragm and the pump performed as expected in a bench test.
Further testing as described in my previous post indicates a pump lobe that is less than spec (having measured another cam eccentric).
Not sure what else can be done other than removing the cam and measuring it. There's too much of the driving season remaining for me to dissable the vehicle for any length of time. A tear down is scheduled for the winter (cylinder head work, new flexplate) so a thorough inspection will be done at that time.
As for throwing money at it...that's was I was trying to avoid by ordering the same pump a 2nd time and getting warranty on the 1st one. Seems now that I have two perfectly functioning pumps in my inventory. The Holley Blue pump and regulator were already on the parts shelf from an abandoned nitrous project so the cost is minimal.

Sonix

iTrader: (1)

Although unlikely, your testing is pretty solid. If/when you do take apart the engine this winter, i'd love to see a picture of a rounded fuel pump eccentric on a camshaft. It'll go in my collection of WTF pictures...
I vaguely remember having a similar problem, but it turned out the return spring on the pump wasn't working, so the fuel pump rod was just resting "full out", and not riding against the cam lobe. I thought this could be your case, but since you've tested the pump by hand, and measured the fuel pump rod travel (1/8") - I think you've pretty well narrowed it down.
Weird though!

Damon

You've obviously done your diagnostic homework so there's only so far I'm going to challenge you on this.

Does the fuel pump pushrod slide easily in and out of it's hole? It should.

No damage to the pushrod but a flat eccentric lobe? Hmmmm...... OK..... maybe. Seems very unlikely only one side of the equation would go to pot while the other side would be fine, but I guess it's possible.

I couldn't find the Edelbrock 1721 pump on their website. I know the older pumps were famous for crapping out unexpectedly- they were cheapies. The newer pumps are made by Carter (hefty cast aluminum body) and should be as rock solid as the Carters (which are very reliable pieces- I have one still going strong after 12+ years withough a rebuild on my Malibu).

I'm interested in hearing what you find. Maybe you did kill a fuel pump eccentric. Would be the "one in a million" but maybe you did.

skinny z

That it is. It seems I'm on my own with this one. Interestingly this is the same engine (albeit COMPLETELY rebuilt since) that ate a flat tappet cam. Wiped out half a dozen lobes.

I put the 1st 1721 pump (yes they are still on the Edelbrock website and made in the USA!) to replace a Carter that I ran for a few seasons. (Went 12's with that one too). It started to act up (or so I thought) and I replaced it. At the same time I installed a Moroso light weight push rod. When the 1st Edelbrock crapped out, I went with a 2nd one (chasing warranty). The 2nd "failure" brought about an examination of ALL the components. The push rod looks to be in fine shape. No restriction in it's travel in the block. No galling or obvious signs of distress. Even the OLD pushrod looks as good as new.
I'm not sure what to think however the planned teardown during the off season should reveal the facts.
In the meantime, I've roughed in the replacement Holley Blue pump (LOUD!) and show good fuel pressure. Time to try and break it.

skinny z

It seems a flat pump cam eccentric isn't THAT uncommon after all. While I haven't read any posts in some of the other on-line forums, my racing buddy has.
If I get around to finding some of them, I'll post a link.

skinny z

Just a follow up. I've installed a Holley Blue pump and regulator. That thing is noisy! I can't imagine keeping it however all issues reported previously are now gone.
When I do dissasemble this engine in the off season and find that the cam lobe is wiped out (100% sure that it is) then I'll have to decide on a replacement cam (more power!) and probably a roller tipped pump push rod along with my mechanical pump.
Here a few links to articles that touch on pump lobe wear.

http://speedtalk.com/forum/viewtopic.php?f=1&t=21412

http://www.offshoreonly.com/forums/g...sive-ware.html

http://www.chevytalk.org/fusionbb/sh...hp?tid/201721/

DeathStarr89

iTrader: (1)

I put an Edelbrock mechanical pump on my old 355, it wiped the pump lobe off of my Comp roller cam (XR270HR) in a day. My pushrod was perfect, it was just the pump that caused it.


I too went with a Holley Blue electric, it performed great for years after. Some thick rubber isolators between it and the car shut it up a little but it was always pretty loud.

skinny z

Well, there's some comfort in knowing I'm not completely alone.
Interestingly, this problem started to show with my old Carter fuel pump and a momentary "stall" about mid track at the drags. I put that down to vapour lock after sitting in the staging lines for half an hour. Then at the end of last season (after spending most of the year putting a new chassis together) it progressed to a complete stall when chugging up a long grade (3000+ rpm for 30 seconds or so). Then on to all the troubles listed above.
At least I'm off to the races again.