So, thanks in large part to the Chinese Communist Party virus, my 185 has been pickled and in storage for over a year. While there I had some mods made to it and I told my mechanic about an annoyance where the fuel system was too rich when landing. Fuel injection unit came off and went in for repair, a couple of sliced o-rings were replaced. But now it has a new problem.

When doing a run up at about 3/4 throttle the engine dies. CHT/EGT uniformly fall off. I don't touch anything and at about 400rpm the engine will start to run again coughing and sputtering and then run back up smoothly and then the same thing happens again, and again, and again, and again.

The only way to achieve full power is to run the emergency boost pump but it over riches the engine as one would expect it to do with a working mechanical fuel pump.

Here is something of interest. The 3/4 throttle position where this occurs is the same throttle position were the electric fuel pump comes on IF the High boost pump switch is on (which it isn't).

Anyone seen this with a 550 before?

Hey Phil. Does you fuel pressure drop off before needing to switch on the pump? Or is it building fuel pressure at all? Sounds like it's possible that your mechanical pump is failing.

On the fuel injection unit with sliced o rings. I think you are just talking about the valve unit where your mixture cable attaches and it has another linkage that goes to the butterfly where your throttle attaches?

If the o rings were coming apart you could have some chunks down stream from your mixture valve. everything needs to be taken apart to clean from the line to the spider and to the injectors themselves.

The Cessna fuel flow instrument is not really a flow instrument, its just a calibrated pressure gauge. you can be showing flow on that even if all of your injectors are plugged 100% for example but like A1Skinner says it is useful to check for pressure at the spider.

Also there is a service bulletin that must be followed to set up the pressures for the system I can't recall the number off the top of my head but it is important along with good calibrated test gauges.

What he ^^^ said. Correct Fuel pressures are essential on these engines, and not every mechanic knows how to set them up.

Secondly, when you talk about the point where the electric pump kicks in, I assume you’re discussing the use of the electric pump with the switch on low boost? That’s the switch position which has a mechanical connection to throttle position. The high boost (red rocker switch) sets the electric pump to high speed continuous, and is limited to five minutes.

The following has nothing to do with your problem, probably, but it’s essential to understand the function of the fuel pumps in these airplanes, so a review:

The engine driven fuel pump runs at speed (and flow) dictated by the engine’s rpm. Simple: higher rpm, more flow.

The electric pump, on the other hand, has THREE speeds only:

Low boost, with yellow boost switch on and throttle between idle and about 2/3 open. This position allows the engine to run at or close to idle in flight if the mechanical pump fails.

Medium boost, with yellow rocker on and throttle open. This speed works continuous and is designed to keep the engine running if the mechanical pump failed, and you’re in flight.

And High boost, with red rocker switch On. This position permits the engine to make max power with the mechanical pump inop.

The reason for the five minute limit on the high boost position is a cooling issue. That pump gets REALLY hot when run high boost. Which is also why the switch is spring loaded. Look at the location of the electric pump....it gets and needs a lot of cooling.

My priming procedure for first start of the day is: Master on, Energize yellow rocker switch...low boost, and listen to the sound of the electric pump. Now, push throttle to wide open, and listen for pump speed increase. Now, momentarily energize high boost (Red rocker switch), and listen for higher speed of pump.

Now, throttle to idle, boost off, and start. This procedure verifies all three speeds of the boost pump are available. If the mechanical connection between the throttle and boost pump is disconnected, in this condition, if the mechanical pump fails (rare, but happened once to me....over Shelikof Straight ten miles from shore), you will only be able to run engine at cruise power using the emergency speed of the pump (red rocker) and they only guarantee the pump for five minutes at that speed.....

I know, more information than you want....and you likely already know, but....

MTV

Thanks for your ideas guys. Here is where we are at. We isolated the fuel system from the plane using a gas can on a ladder. No difference. Screens are clean, spider and lines are clear as are injectors. The fuel injection shop that worked on it asked us to check the unmetered fuel pressure. We did it was 27psi. They told us to adjust it on full turn which we did, that got us to 37psi. That allowed the engine to get to 2600 rpm before it dies instead of 2000 rpm. When it dies it’s like pulling the mixture all the way out as fast as you can.

just replaced mine and wondered why the hell it was ducted like it glows red. Hardly run it for long even for hot starts. Thats why. What about gravity feed though? Is it enough to run it at all?

frstnflt wrote:just replaced mine and wondered why the hell it was ducted like it glows red. Hardly run it for long even for hot starts. Thats why. What about gravity feed though? Is it enough to run it at all?

When my engine driven pump checked out, the engine quit cold. I don't know if that's normal, but it sure got my attention.

MTV

Barnstormer wrote:Thanks for your ideas guys. Here is where we are at. We isolated the fuel system from the plane using a gas can on a ladder. No difference. Screens are clean, spider and lines are clear as are injectors. The fuel injection shop that worked on it asked us to check the unmetered fuel pressure. We did it was 27psi. They told us to adjust it on full turn which we did, that got us to 37psi. That allowed the engine to get to 2600 rpm before it dies instead of 2000 rpm. When it dies it’s like pulling the mixture all the way out as fast as you can.

Wow, that is weird. Many shops tend to set the unmetered fuel pressures too low to start, which isn't good. But that is a very different problem.....and scary.

Good luck with it.

MTV

Problem found?

Nope, the plane is still grounded. Everything (that can be thought of) has been inspected, cleaned and/or replaced. Continental has even gotten involved. Next step will probably be to replace the fuel pump- again- just because. When we do finally get it figured out I'll let you know what the culprit was.

Thanks.