Posted: Sun Apr 08, 2012 8:52 am Post subject: M38 fuel pump vs Cam shaft
I have a problem with the fuel/vacumn pump on my M38. Whats new!?
When I bought the M38 it had an electric civvy fuel on it. Then I bought a rebuilt, installed it and it did not have any vacumn. I pulled it off and sent it off to someone else to have it rebuilt. When I reinstalled it same thing. It runs well but again no vacumn. Sent it back and haven't received it back yet but the rebuilder says it bench tested O.K. but he stated that the camshaft may be in poor shape! Can the camshaft be in such a state that the pump will pump fuel but not vacumn?
Any help would be appreciated.
Thanks _________________ jackthebear46
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The camshaft fuel pump drive cam would have to be seriously worn and weather it could then support ptoper fuel delivery is doubtful.
Have you taken actual fuel pressure readings while the rebuilt pump was installed? They would have indicated very low if the cam was worn enough to cause the vacuum side of the pump to not function.
Are you sure the vac side of the pump was plumbed correctly?> _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Thanks for the reply. Sorry I didn't reply sooner as I'm just getting ready to reinstall the second rebuild. Re the plumbing, the line from the intake manifold goes to the outlet closest to the block & the line from the w/w & distributor goes to the outermost inlet? Is this correct?
Thanks in advance. _________________ jackthebear46
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Thanks again Wes.
I just came in from installing the second rebuilt pump. The lines are plumbed correctly as per you picture. Same problem, no vacumn.The pump was bench tested by the rebuilder and everything was O.K. I guess I have a camshaft problem. _________________ jackthebear46
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Are you getting rated pressure from the fuel side of the pump? _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
A bit of an odd question from a fellow troubleshooting fuel/vac pump issues for the last 14 days!
4 to 5 1/4 PSI @ 1800 RPM
The reason for my question is if the cam profile actuates the pump lever adequately for the proper fuel pressure then I fail to see why the vacuum output is not good as well since it relies on the same pump lever.
The probabilites of cause if fuel pressure is good are:
1-Vac side of pump bad!
2-You are attaching your vacuum gauge to the wrong pipe.
3-You have a plugged piece of tubing.
I recall asking the same question about fuel pump pressure in my post 12 days ago
Wes, I wasn't troubleshooting a fuel pump problem for the last 14 days. I had a vacumn pump that didn't work , sent it back and it took 2-3 weeks to get the 2nd rebuild back, put it on today, still no vacumn. Never was fuel pressure an issue. I guess we were on a different wavelength. _________________ jackthebear46
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Might be. When I install a new fuel pump I check it's output with a gauge no matter why I changed it. Same with a Fuel/Vacuum pump. If I hang a new one or rebuilt one on mine or a customer's rig I check the output of both functions with a gauge. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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