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Engine was ideling at 900rpm up from 600rpm. then would idle…

Customer Question
engine was ideling at...

engine was ideling at 900rpm up from 600rpm. then would idle rough. 2001 duramax.

Mechanic's Assistant: Is the rough idle intermittent or consistent?

Consistant, but will run if given fuel.

Mechanic's Assistant: What is the model of your '01 Chevy?

2500HD Eura 2500HD Duramax

Mechanic's Assistant: Are you fixing your vehicle yourself? What have you tried so far?

Haven't tried anything yet. will fix myself if possible.

Submitted: 7 months ago.Category: Chevy
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Answered in 14 minutes by:
11/29/2017
Chevy Mechanic: Pete, Auto Service Technician replied 7 months ago
Pete
Pete, Auto Service Technician
Category: Chevy
Satisfied Customers: 32,981
Experience: 14 years GM dealer Technician.
Verified

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

Is or has the check engine light come on with this issue here at all?

Was anything checked/tested or replaced to try and locate or repair this fault?

Let me know,thanks Pete.

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Customer reply replied 7 months ago
nothing checked or replaced. engine light was on 100 mi. ago but went off.
Chevy Mechanic: Pete, Auto Service Technician replied 7 months ago

Was the issue occuring 100 miles ago when the light came on?

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Customer reply replied 7 months ago
no
Chevy Mechanic: Pete, Auto Service Technician replied 7 months ago

Is the engine oil over full at all or smell like diesel fuel?

Does it smoke excessively with this issue?

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Customer reply replied 7 months ago
no excessive smoke or fuel in oil
Chevy Mechanic: Pete, Auto Service Technician replied 7 months ago

O.k.,without a check engine light and it doesnt sound to be injectors I will have to open this up for others here to see if anyone else can assist further on this.

I will open your question up for others here to see if someone else can assist further on this as there may be others here that may have more info on what to check or do with this issue.I hope you can get this resolved with ease.No need to reply to my last post here unless it is a last resort as it will keep your question locked to me and others will not be able to view it.If you feel you want to continue to work with me on this feel free to reply at any time but keep in mind in some cases it can be difficult to fully diagnose or help repair this without seeing your vehicle.

Thanks Pete

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Chevy Mechanic: Matt, Engineer replied 7 months ago
Matt
Matt, Engineer
Category: Chevy
Satisfied Customers: 23,314
Experience: Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
Verified

Hello

another expert here

I'd say that this sounds like an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propaneor brake cleaner around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.
1 vacuum leak path that you won't find with the above test is if the brake servo is leaking

so try clamping off servo hose to see if this has any effect

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.

if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

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Chevy Mechanic: Matt, Engineer replied 7 months ago

Hi
do you still need help?
Bear in mind that the site takes a deposit from you at the beginning and this is held by the site until you rate my answer at which point the cash is split between the site and the expert. I am only paid for my work on this question if you rate my answer, using the star system at the top of the screen. Please do not forget! If you're not pleased, also let me know because rating bad does not result in a refund, there are better options. Rating now will not close this post nor prevent further replies
Thanks Matt

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