Boost jump - Diesel Forum

MyT · 04-04-2008, 07:59 PM

This is first post. I hope someone can help. While driving at constant highway speed with the Banks Sidewinder boost between 3 and 5, the boost will jump to 10 or more. If I keep the accelerator pedal in the same position, the truck (89 F350 7.3L) continues to accelerate. When I back off the accelerator, the boost drops somewhere around 5. Then, in a few seconds, it may drop lower. At that point the engine runs normal. This may happen every few minutes or it may not happen at all on a 30 mile run. Sometimes when I back off the accelerator, the engine/truck bucks a couple of times.

Kaliburz · 04-05-2008, 02:12 AM

And for a moment, I thought this was an ELECTRICAL question....

My wild shot in the dark..... waist gate? (If it has one.....I know an ATS system does-buddies truck has an ATS.)

MyT · 04-05-2008, 07:38 AM

Thanks for responding. I'm quite sure the problem is not electrical. The turbo does have a wastegate which seems to function normally most of the time. I wondered if the control input to the wastegate is causing the problem.

94_turbo · 04-05-2008, 03:28 PM

either that-------OR------your bearing is getting weak in the turbo--and oil is getting into the engine--and its starting a runaway---engine use oil????

MyT · 04-05-2008, 04:14 PM

Thanks for the response. Engine oil consumption has not increased. The turbo is relatively new, less than 20K miles. I will watch oil consumption and consider the possibility of a bad bearing.

7.3L F-250 · 04-06-2008, 12:46 AM

maby the waistgate is sticking

i know mine is stuck shut

MyT · 04-06-2008, 09:35 AM

Thanks for the response. Any ideas on how to lubricate the wastegate to prevent sticking? It seems like a liquid like WD40 would quickly burn off.

94_turbo · 04-06-2008, 09:05 PM

if it could be somehow worked in the shaft area--antisieze would be great--but aint sure how youd do it---

88blackidi · 04-06-2008, 10:12 PM

i dont think its the waist gate. at rest and under normal operating conditions the waste gate is in the closed position and held there by the vacuum can. it only opens to dump exhaust gas when the boost reaches 10psi or so and the vacuum is right to do so. this is why to squeeze a little more boost out of the turbo u can change the length of the actuator rod or even take it off, but i dont recommend this. guy from banks said u can probably only pull 12psi of boost or so by unhookin the actuator rod. back to ur problem the waste gate only opens to dump boost, a stuck gate in the closed position would be hard to notice unless u have excessive boost and a gate stuck open would give u no boost at all. id check for a fuel related problem, dirty filter, inadequate fuel supply pressure, maybe a pinhole in the tank pickup line...causing just a little air suckage? does it have this problem on both tanks? full tank? starts after u run one of the tanks down a bit? im puttin my money on a fuel related prob and not the turbo. or its eatin some oil like 94 turbo said. good luck

arthurgoboom · 04-06-2008, 10:13 PM

Could something in the IP/governor cause it to inject more fuel than normal at some times? The boost jumping to 10 sounds like it's just getting more throttle at random intervals. It does indeed sound like a runaway scenario of some sort. Is there an overabundant amount of oil coming through the CDR valve where it attaches to the Banks intake?

I don't think the wastegate is sticking, since it would probably boost beyond 10psi if the gate was stuck.

MyT · 04-07-2008, 11:29 PM

Thanks to 88blackidi for your response. It helped me better understand how the wastegate works. I also had wondered about fuel supply problems, especially during one boost jump episode when the fuel filter light came on momentarily. I changed the fuel filter, even though the one replaced was not old. Filter replacement didn't stop the boost jumps. Supply pressure and pressure have not been tested. It seems to me that air sucked into the supply line would accumulate in the top of the fuel filter. Although I haven't tried this yet, it seems that the air could be manually vented from the fuel filter through the Shrader value. I do have fuel tank issues that may be contributing to the boost jump in some way unknown to me. Most recently, I discovered that the fuse for the fuel tank switch was blown. That wasn't obvious to me because the fuel gauge has never been accurate for either tank. So, I not have tested both tanks recently. To make matters worse, I can use only about 10-15 gallons out of each tank before running out of fuel. I wondered if the pickup tubes are bent or if the wrong pickup tubes are installed. Question about wastegate operation: If the wastegate is closed during normal highway operation, what causes the boost to jump quickly when the accelerator pedal is floored?

MyT · 04-07-2008, 11:34 PM

Thanks to arthurgoboom for your helpful response. I don't understand the purpose of the CDR value or why excess oil will cause the boost to jump. Please explain. Also, I don't understand how a mechanical IP/Governor could cause a boost jump that stops shortly after backing off the accelerator pedal. Any sharing of your knowledge will be appreciated.

arthurgoboom · 04-08-2008, 12:36 AM

First off on the 10-15 gallons of fuel before running out, the fuel pickup tubes in these trucks are known to fall/break off, so you can only draw the tanks down to about 1/4 before they start sucking air, and the remaining fuel is stranded in the tank, so that is most likely the case in your truck.

As for the CDR valve, it stands for Crankcase Depression Regulator. It's just a diaphragm valve in the crankcase vent line. It's supposed to suck shut and not let the engine drink crankcase oil if the air filter becomes restricted. If someone leaves a rag in the air filter or something, it can start blowing engine oil up into the intake, and the engine will burn it and run uncontrollably until it blows up from high RPM or runs out of engine oil (which usually means the bearings run dry, and it trashes all the bearings in the process). I've seen it happen on pulling tractors...whoops. Under normal operating, it's just a straight shot of crankcase vapor going back into the intake to be reburned. You might be getting enough oil every now and then to make the engine accelerate some.

Back to the boost issue, anytime you give the engine more fuel (throttle, excess oil, ether, whatever) the engine will in turn make more exhaust gas, which will turn the turbo faster, which will make more boost pressure. The wastegate is controlled by boost pressure, it stays closed until pressure reaches 10psi or so, then it opens to not exceed that 10psi maximum. All it does is limit the maximum boost. The gate is closed at any pressure lower than 10 psi. My theory is somehow the engine is getting more fuel from somewhere, causing it to make more exhaust gas and thus more boost. I was just wondering if any kind of failure in the IP can cause overthrottling, I'm really not sure of that.

Just a thought...I have read on here before about a guy having a fuel lift pump fail, and it leaked diesel fuel into the crankcase, then it got so full that eventually it started pumping the fuel/oil mixture through the CDR valve and the engine ran away. Check your engine oil and make sure it's not overfull. Dipstick is designed to be checked hot (since the oil expands when hot) after the engine has sat for 10-20 mins for oil to drain back down.

MyT · 04-08-2008, 08:18 AM

Thanks, arthurgoboom, for your very informative response. I am very pleased with the information provided to me through this forum. I feel I know enough now that I may be able to identify and fix my boost jump problem. Unfortunately, my employment will keep me away from home for several weeks so I probably won't have any reports of success or failure.