rhinoo

12 hours ago, fjh said:

So I have a suggestion for you Before you go any farther ! Those numbers Aren't that bad seen alot worse if that engine is maintaining 25 psi plus under load the charge pressure is OK!

REmove the 3/8 plastic line that goes to the smoke limiter at the back of the pump at the top suck on the the tube if it won't hold a vacuam  the diaphram in that device is fubar you need to go there ! Remove the four screws you'll likely find your problem!

Just sayin!

So I re read the other posts and eleven under load is to low for sure your heading the right way fuel transfer pump or a leak prior to it! also check your tank vent! The above check is worth doing if you have good fuel pressure !

 

Thanks for the reply. I didn’t check that the way you said but yesterday I disconnected that line at the intake and applied a little bit of shop air with a rubber tip blow nozzle and held it there for a few seconds and when I pulled it off the tube it was still holding air pressure. I may try what you said tomorrow though. Did put a new lift pump on today and still have same exact pressures as I had before. Back to the drawing board. 

3 hours ago, Vmac3 said:

Yes are absolutely correct, but we already have low fuel pressure to begin with. The poster already mentioned that they checked all basics. So I don't see as to why the poster would have to check it under load to reconfirm that there is low fuel pressure. The only other thing is to bypass the complete fuel system and run the fuel system in a bucket straight from the transfer pump. If the pressure doesn't go up then we can confirm that the lift pump is defective.

V

V

Thanks, I was thinking the same thing and didn’t see why running it under a load would raise the pressure at all but it was an expensive pump and easy to reinstall to try it. I will try to run straight from a bucket of fuel tomorrow to see what we get. 

10 minutes ago, Vmac3 said:

The specifications that I gave were, no load idle and rated speed. These specs are published by Mack for this model year.

V

Thanks guys. Took it for a road test and still low pressure. When making boost pressure would drop to 11psi and when let out of the throttle it would bounce between 20-25psi

5 minutes ago, br549 said:

There is a line that comes from the intake manifold around front of engine to back of injection pump.  As the turbo pressure comes it put pressure to the back of the pump to open up fuel rack.  If the hose has a hole in it or the diagphram in the back of governor it cannot let the rack open up.  We made up a fitting and gauge to put air pressure on it to see if it would hold air  around 30 psi, if it leaked down it could have a hole in the line or the diagphram in the pump which can be easy change without removing pump.  W also used to adjust it to pick up faster if it was just slow response pickup.  You pulled the plug and there is a cog like a brake adjuster in it you take a short screw driver and click it but I can't remember which way to go.

This wouldn’t reflect lift pump pressure though right? I don’t mind to look into that but the lift pump pressure is half of what it should be apparently. Thanks 

6 minutes ago, Vmac3 said:

Have you replaced the fuel filters?. If you have make sure there is no junk in the fuel tanks. Squeeze the return line from the pump to the tank and see if the pressure goes up. If it does then it may be a fuel return check valve. If not then the fuel pump may be no good.

V

Ok, it has new filters and tanks were clean. I will pinch return off to see if it changes. I did remove the pump and the cam in the injection pump looks good. 

41 minutes ago, Vmac3 said:

That is a very reliable engine. Don't expect the truck to be a rocket, but it is very good.

V

 

Pressure is checking low. I guess I should try a lift pump? The pics are of idle and wot. 

24 minutes ago, Vmac3 said:

Oh shoot. I just noticed the stop lever on one of the other pics. 

That engine is an E7 mechanical. Fuel pressure should be 23-39 psi.

My apologies.

V

Ok, thanks. Is the mechanical a good thing or bad? Trying to get teed into fuel system now but gotta go pick up a fitting. I talked to a guy at Rochester diesel earlier and he said that if someone put that s400 on without upgrading the injectors that it would be sick. I told him I was gonna check fuel pressure first and if it was ok I would call him back and order a set of injectors. 

43 minutes ago, Joey Mack said:

V-Mac 2

I’m looking at truck now and has no ecm, manual foot throttle, with Bosch inline pump.. I sat on phone with Kentuckiana Mack for 30 mins for them to tell me it was an e7-350. 🤦‍♂️

41 minutes ago, Vmac3 said:

May be a governor setting. I would hook up the service software and check to see if the gov is set to min/max rather than all speed.

V

Also wanting to check fuel pressure today. Do you know what it should be coming out of lift pump? Thanks

38 minutes ago, Vmac3 said:

May be a governor setting. I would hook up the service software and check to see if the gov is set to min/max rather than all speed.

V

Ok, is this engine a vmac l or vmac ll? Or something different?

Ok, I will check to be sure check engine light is working when I key up in the morning. I did already change the air intake boot on turbo because it didn’t look real good when I serviced it. I was looking all of them over closely earlier tonight and there is another one that doesn’t look great either but haven’t checked for leaks. Is there enough pressure when idling that soapy water would show a leak? Vin# 037403… if that turbo happens to be upgraded I guess I would need to check injectors to see if they are upgraded also. Should this truck at least puff a little black smoke? Especially with lugging and turbo not spoiling up?

3 minutes ago, rhinoo said:

I did do a full service on it once I got it home. No check engine light. 

 

17 minutes ago, Onyx610 said:

Any chance you can put a gauge on it and get some boost pressure readings? Might be a good place to start. Along with fuel pressure like you said. I believe it’s 70-100psi

Yea, I think that is the next step if the turbo is correct. Can I tap in at the inlet on secondary fuel filter? 

16 minutes ago, R.E.D said:

I believe that truck should have a s300 turbo.

Have you replaced fuel filters ?any fault codes ? lightning ⚡ bolt on?

I did do a full service on it once I got it home. No check engine light. 

Got a new to me 98’ 688s with an e7 350. I noticed a slight turbo lag when I test drove it but once I finally hauled a load the lag was worse. It seems to take a few seconds to get under the turbo before it picks up. No smoke at all. I actually choked the motor out twice down shifting going up a hill.  The shift was made in plenty of time but the turbo delayed so long the motor kept going down. The second time I tried to drop 2 gears but it still didn’t pick up in time. If I keep the motor wound up it does decent. I own several trucks and drove them all, even a Mack 96’ 613 with an e7 427 and it doesn’t fall on its face like that at all. So, I was thinking maybe not getting enough fuel but I was also wanting to make sure it has the correct turbo on it. Does anyone know what turbo should be on this motor? I was thinking tomorrow I will also try to check the fuel pressure. Does anyone know what it should be?