M1078 runs slow

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Coastie54

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Our M1078 is currently speed limited to about 25mph. Had a huey pump issue and replaced it. Is there a speed sensor somewhere limiting the speed?
 

doghead

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Huey pump?

Did you mean,
Heui pump?
 

Coastie54

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Yea, typo. Heui pump. Replaced it because other one went bad and truck would not crank. Got it running again with new pump installed at a diesel shop though but later developed another issue of truck running slow and speed limited.
 

snowtrac nome

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did they flush the fuel and oil rails, when the heui pumps go bad they send metal through the fuel and oil systems if you have a heui failure you should also replace injectors and flush the fuel and oil rails.
 

Coastie54

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Yes. All flushed but did not replace injectors. Having a friend join this thread soon who coordinated repair who can explain why injectors were not replaced this time.
 

MacGreenie

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2003 LMTV 1078 A1 with 3126 with 12000 miles on it............

We understand very well the issues with HEUI pumps littering the system with metal fragments and on our inspection after HEUI pump failure we could find no metal in the injector areas or around the injectors or under the valve cover. Of course we did the complete flush to make sure nothing was in the galleries or the fuel lines. That was about 1000 miles ago. In the meantime we replaced the throttle position sensor and the IAPCV valve. No luck. Yes we have checked all the grounds and connectors and the only thing we haven’t done is go to the local CAT Dealer and spend the money to have them put it on the ET system. That appears to be quite possibly the next logical step.

Our mechanic examined the injectors and couldn’t find any scoring or evidence of any metal intrusion so we continued using the current set of injectors.

Our problem is a check engine light comes on and appears to de-rate engine and does not allow it to accelerate. It’s almost as if the last 50% of the throw on the gas pedal is dead and the truck only wants to get to maybe 40 miles an hour. If you back off a little bit on the speed and then try to accelerate again every now and then you can punch through this 40 mile an hour barrier and the truck will go right to 58 miles an hour and run smooth all day long.

Turbo builds Boost and works as advertised. The vehicle has had a recent oil change within 1000 miles and all new filters all around and the system has been traced down to eliminate any air leaking in on the suction side of the fuel system. We even added a new fuel primer bulb as we know that those are a likely source of failure for air leaks.

When it runs it runs really well and occasionally it will accelerate without throwing a check engine light and have all the power in the world.....And then the strange de-rating comes in and you can’t get past 40 and on alternate the attempts at acceleration sometimes you can’t get past 25 miles an hour.

The transmission is not throwing any codes.

I know , I know , take it to the CAT dealer and have them look at it with CAT ET.

I just want to know if there’s anyone else out there who has experienced this weird behavior? I know a lot of people in the RV forums have experienced it but I don’t know if the LMTV crowd has seen the same thing.

Thanks in advance for any and all of your valuable input!


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Suprman

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I have seen a lot of the pickup fuel lines go bad. I have tried to pressure test them for leaks and it doesn’t work but replacing the line and back to normal. If you have a voltage drop issue that could cause prioblems. Some trucks have a box between the power feed and the engine computer. It’s a noise filter but they have a habit of going bad. Usually they short or open though. You can pull basic engine codes with a generic heavy duty code reader.
 

snowtrac nome

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you can spent a lot of money replacing things to avoid using the et get a code reader like a pro link or pay some one with e-t to plug in to it the engine could derate for anything from a coolant temp sensor to a crank sensor you will have to plug in to find out why.
 

Coastie54

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Replaced the HEUI pump last month. Still runs slow. Took it to CAT yesterday. Said bad HEUI again. Replacing with CAT HEUI. Will know in a few hours if THAT fixed it. At least that’s what the tester is saying it is.
 

MacGreenie

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To finish Coastie54’s input here’s what we did today:

Problem solved: replaced HEUI pump (for the second time in only 1500 miles). Apparently our first factory reman HEUI unit was faulty.

While troubleshooting without benefit of CAT ET software diagnoses we replaced the following:

IAPCV valve
Throttle position sensor

These seemed likely culprits for derate condition based on what RV owners have experienced with CAT 3126 series engines.

Engine continued to throw check engine light during acceleration through 35-42 mph range and would derate engine performance every time we tried to go faster than 42 mph.

Once we got to a CAT service facility, the first ET indication of a problem pointed to IAP Sensor for injection oil pressure readings. CAT ET showed IAP erratic code which could be attributed to low sensor signal, excessive oil return from injectors or weak HEUI pump. So we replaced the sensor and this still had no effect. On the up side, noticed oil leak from sensor o ring that had deformed so by replacing this sensor and o ring we eliminated a pesky and mysterious oil leak.

Today replaced HEUI pump at CAT dealer and she runs like a champ. No CEL indications. Much more power and more boost from what my ears tell me.

FYI: after first HEUI failure we flushed all oil galleries. No metal noted. Kept same injectors in service after removing them and inspecting them. No metal noted on second HEUI partial failure so kept original injectors in place. Runs fine.

Just wanted to close this issue out and not leave the thread hanging.

Thanks to all who responded.

Be careful out there and thanks for your service...


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MacGreenie

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Thanks to Snowtrac Nome and Superman for input.

We chased down all the things you guys mentioned. Good points and we eliminated them all. CAT ET was only option in the end game and local CAT dealer treated us well on labor.

Yep. You are so right. Without CAT ET or a good scanner you’re just in the dark on these 3126 engines. My first mechanic could not pull up IAPCV related codes so his scanner system might not have been very robust.

So we are back in service and we can enjoy our truck at last. Hooray!


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