Slow Acceleration / Reduced Top Speed - 1981 DD 8V71

[HenryTX]

The other day I took it out for a spin.
Though normally starting right up with no issues, this time it started up very slowly.
With the pedal to the floorboard, acceleration was slower than normal.
Top speed was 50 mph, when I've hit 70 mph in the past.

My fuel filters were changed less than 5000 miles (less than 2 years) ago.
Regardless, I called MCI to ask what might cause problems with the fuel pressure. Though he referred me almost instantly to Detroit Diesel for any questions about the engine's fuel system, he did also suggest that I check the color of the diesel fuel in my filters. If it was greenish, he suggested, then the problem could be algae in my fuel tank.
Upon following his suggestion, the diesel fuel in my filters looked like the normal yellowish color.

Since that last outing, I've replaced the air intake filter (dry kind, not the oily kind), and I've started her up a few times.
She starts up fine, but I haven't taken her out for a spin yet.
Each time I've started her up, I've taken readings from the fuel pressure gauge inside the engine compartment.
At start up, the pressure gauge reads 50 psi. After idling for a while, with a few manual accelerations at the engine, the pressure gradually drops down to 25 psi or below.

Anyone know what the fuel pressure should be while idling?
Has anyone experienced anything similar?
Any ideas what could be causing this weaker performance?

[buswarrior]

Fuel pressure gauge in the back?

sounds more like oil pressure behaviour?

Trace that line...

What condition did you find the air filter in that you changed out?

happy coaching!
buswarrior

[DoubleEagle]

The fuel filters can get blocked up quick with one load of contaminated fuel, and there is always the possibility of the fuel pump giving up. Check the air filter just to be sure, but the fuel filters are a good bet.

[luvrbus]

You can have good fuel pressure and a filter be plugged it all depends on which side of the filter the gauge is reading from,  change both filters and you can roll again

[Geoff]

If your exhaust is smoking black under load, it is a plugged up air filter (bird nest?), If not, it probably is plugged fuel filters.

[eagle19952]

could be a linkage problem, getting full travel ?

[richard5933]

5000 miles and two years on a set of fuel filters? That would be my number 1 place to look for the problem.

When I brought my bus home from NJ last fall, it had recently had new filters put in. By the end of the 1300-mile trip home, I was noticing a reduction of power on the hills. Sure enough, the primary was pretty dirty and the secondary starting to get there. Changed them out and all was good again. Doesn't take much to plug them up enough to affect performance.

[luvrbus]

Check the shut down cylinder too and be sure it is retracting all the way and not hanging 

[chessie4905]

Let's see.....1981 coach with the build up of crap, dirt, algae and moisture in fuel in fuel tank over the years, which settles to bottom of tank, only to be stirred up at every refill, and sloshing fuel going down the road.....change, at least primary fuel filter. Pre fill it with fuel, install, start and run engine immediately at 1500 to 1700 for 15 to 20 seconds, before dropping to idle so you don't allow the little bit of air introduced when changing filter/s to cause engine to stall out.
The majority of older conversions should have fuel tanks pulled and thoroughly cleaned. Will save many fuel filter replacements over the years. But most won't do it.

[HenryTX]

Thanks for the quick replies, everyone!
I will change out the fuel filters and let you know how she performs.
I will also plan on pulling out the diesel tank and washing it out.
What's the best way to wash out the fuel tank? Just hose it out?

Fuel pressure gauge in the back?

sounds more like oil pressure behaviour?

Trace that line...

What condition did you find the air filter in that you changed out?

happy coaching!
buswarrior

I have an oil pressure gauge on the dash (shown in attached photo with master switch in "off"), and there's a different gauge in the engine compartment that's connected to what I think is the fuel pump, but I'm not sure (i'm totally a newbie).
When the bus is running they show different pressures, so they can't both be gauging the oil pressure (unless my dash board gauge is innacurate)
The other pictures show the fuel(?) gauge in the engine compartment and what it's connected to ("blue component").
I'm assuming it's the fuel pump as it's also connected to what I think are fuel lines.

But, I'm going to download the Detroit Diesel 8V71 manual to start reading up on my engine anatomy.

As for the air filter, it was pretty gray though not crazy dusty. But, I know it hadn't been changed in several years. Looking on the inside of it, no light was permeating through the filter walls (like the new one that I ordered).

[wildbob24]

The "blue" component in your last photo is the air compressor that supplies air pressure for your brakes, suspension and wipers. If the hose from the manifold that that gauge is attached to is the small one attached to the compressor, it is supplying engine oil to the compressor crankcase, so that gauge is oil pressure, as well. It's not unusual for the 2 gauges to disagree.

Bob

[eagle19952]

Fuel filters reduce flow by gallons thru not hours or miles or time.
Yes I know I'm full of XYZX.
Fuel tanks don't make water.
I know, more BullXYZX.
And fuel doesn't wear out..well ok maybe after years, more than 5...
been around stored equipment, mothballed between contracts for years and never found algae or water in a tank that just sat in storage.
And I've owned and serviced equipment that had 4-5 year old fuel filters.
They ran fine.
If it ran fine last week, it probably isn't the filter.

[neoneddy]

I had a similar issue. I had someone push the pedal all the way down and noticed I only got 2/3 to 3/4 travel.

The linkage was bent. I think it happened when I tried pushing the pedal down with the high idle on.

[chessie4905]

When I cleaned the tank in the 4104, I used pea clean gravel, about a coffee can worth, and a gallon of lacquer thinner. Rocked tank back and forth like a see saw. When done, removed solvent and gravel, left tank air out till lacquer thinner evaporated. Reinstalled. I was amazed at the layer of crud in bottom of tank. Keep any flame or sparks away while doing it. Some tanks have baffles that don't al!ow solvent and gravel to pass back and forth, and might need steam cleaning instead. I've just shortened a 4905 tank and had the ability to look closely at the baffle system. The baffles are about every 20 inches, go from top clear to the bottom. About 2 inches up from bottom there is an approx 2" hole to allow fuel to pass. Some fuel can get by at bottom of baffles. Looks like it may have been designed to avoid dirt and debris from moving through whole tank, but be contained to first compartment which has the fuel drain , and big removable fuel filler. Sort of how septic tanks are designed today.lol
Btw, with fuel filler removed, you can see the slot in baffle for fuel gage float clearance, whether it has the fuel gage or not. This tank had some crud in first compartment, but not like in the 4104.

[HenryTX]

I think it's safe to say this issue has been resolved.

I followed the suggestion most of you made to change out the fuel filters.
First, I tried flushing the fuel system with a can of Sea Foam. Then, I changed the filters out and added some fresh diesel with a new can of Sea Foam (1 oz per gallon of diesel).

She revved up beautifully in neutral and when we took her out, acceleration was back to normal and we hit a top speed of 65.

I made an effort to remove the entire tank for cleaning, hoping to avoid having to change the filters again too soon.
But, I was not successful in detaching the fuel lines without potentially damaging them. I will try again some day, but for the time being, I'll continue administering Sea Foam when the tank gets low. Hopefully, that will keep everything cleaner.

The one downside to this procedure is that I may have let a bit of air get into the fuel lines while I was changing out the filters. Immediately after changing the filters, the ignition button struggled to get the engine to turn over. I had to press down on the gas to get it to push through to full ignition and then a normal idle.
I ran it until the air pressure was full, revving it once and a while. The next time I started it (to take it for a spin), I had the same issue. I haven't tried starting it since our little spin, but hopefully that several mile circuit pushed any air bubbles out.

I'd like to avoid having to prime the fuel system, if possible.
If not, any advice on how best to prime?