RD688 Boost

[Jamied625]

I was driving for my dad today he has a Rd 688 with a 300 and a 8LL trans and tops could only make about 8lbs of boost, that seems low to me. Should it be more than that?

Thanks,

Jamie

[Lmackattack]

IIRC you should see about 23-27psi ... does it blow black smoke, low on power? look for a large boost leak in the system

[Jamied625]

no the truck doesnt really smoke any time even during a heavy pull

[Lmackattack]

well with 8lbs of boost and no smoke you either have a bad reading on your gauge or the fuel is cut way back and it will not be pulling like it should. just running down the road at 1800RPM and not throttel its pulling 2psi. as soon as you touch the pedal it would jump up to around 10-15 psi under normal cruiseing speeds

[Jamied625]

ya it cruises at around 1 or 2 but when you get into it it only makes about 8 or so

[dds92780]

Hmm, that is pretty low, Lmack is right though. Your boost should be higher and im surpised it is even able to pull anything. Where is the boost gauge hooked up? With boost that low i wouldnt see it even being able to get past 45-50mph

[Jamied625]

ok i will start by seeing if the guage is bad. The truck is a dog compared to our other trucks we have with the bigger motors, but it does pull up the hills slow but it always tops them.

Thanks for the repleys,

Jamie

[Jamied625]

where should the guage be hooked to?

[Lmackattack]

It should be plumbed into a port in the intake manafold or at some place in the piping between the intercooler outlet and intake manafold . this gives you the true boost reading after the air pressure drop thru the intercooler. Also check the line for kinks or oil blockage. that can give false readings as well

[Jamied625]

ok thanks Lmack i will see what i can do

Jamie

[MACKS]

A quick way to check for leaks is to spray some starting fluid on the all the intake connections while the truck is idling!

[dds92780]

It should only be tapped into the intercooler piping just before the turbo. If it is plumbed into the intake manifold it would be bouncy within 5 psi but that doesn't seem like your problem based on what you described

[rsb502]

It should only be tapped into the intercooler piping just before the turbo. If it is plumbed into the intake manifold it would be bouncy within 5 psi but that doesn't seem like your problem based on what you described

Actually he was right for a good reading on the pressure in the manifold it has to be on the intercooler pipe on the manifold side or the manifold itself, all mine have been in the manifold itself.

[rsb502]

Seems odd to me its making 1-2 cruising no throttle but pegs at 8 every time if it hits the same spot loaded or empty no matter the conditions, I would bet its a bad guage, however if its a big enough leak it may just be able to overcome a crack or hose tension at 8psi then bleed excess over 8 psi. I think per the Mack spec an E6 300 should make 13-21, however in an RD I am guessing its an E7 and I dont have my E7 book anymore and I cant remember all of em, but most E7's make 5-10 psi more per equal hp rating.

[dds92780]

i know e7 350 makes 24-32psi under load (with a load on)

[Jamied625]

ok well i still havent made it over to check it out but i will start with leaks and if nothing there i will go to the guage

Thanks,

Jamie

[rsb502]

A spray bottle full of water with about a teaspoon of dish soap sprayed on the intake system with the engine running about 1000rpm will show any leaks, theres enough positive pressure made even without the boost guage reading anything to make bubbles, thats how I track mine down I dont like the starting fluid method.