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Mysterious whistle under load

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Mysterious whistle under load

Postby ElIsuzu » Fri Mar 22, 2019 7:57 am

I have a 1994 Mu with a 4JG2 - the 3.1L turbo diesel. It has developed a whistle under load, I'm pretty sure it's turbo related as it only happens at certain boost pressures.

Here's what I've tried:
-Briefly ran the engine with all accessory belts removed to confirm it's not belt noise. It's not belt noise.
-Connected my compressor to the air intake just after the air filter to confirm it's not a boost leak. I gradually increased the pressure until it blew the intake hoses off at about 25PSI. No boost leaks.
-Connected my compressor to the exhaust pipe and repeated the same test. I found a few minor exhaust leaks, but they only occur when I pressurise it to beyond what you would expect exhaust pressure to reach. I couldn't make any of them whistle at any pressure.
-Tried to replicate the issue while stationary - it's very difficult to do. I can make it produce a very short whistle if I open the throttle suddenly. It seems to be coming from the side with the turbo, alternator, and aircon pump. It might be the turbo, but I don't want to buy a turbo unless I know for sure.
-I've removed the intake tubing and inspected the intake side of the turbo. It looks fine to me, but I'm not an expert. No significant play in any direction.

What do you think it could be? What else can I try?

My best guess is to pull the turbo off completely and inspect the exhaust side. What am I looking for? What do I need to know about removing and reinstalling the turbo? How much should a replacement turbo cost? Thanks guys.
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Re: Mysterious whistle under load

Postby geeves » Fri Mar 22, 2019 1:14 pm

Soon after I bought my 89 Bighorn it developed a very similar noise. Took a while to track down. Turned out the dealers mechanic had removed the egr and used a very rough piece of Alloy sheet along with the original steel gasket to seal the holes. Replace the steel gasket with a paper one and problem gone
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: Mysterious whistle under load

Postby tonkamu » Fri Mar 22, 2019 3:54 pm

3.1 turbos are bomb's waiting to happen. on a mu its even worse with out an intercooler to stop bits of compressor impeller entering your engine.

Just replace it with a better turbo. they are reliable cheap cars but they won't last forever you'll need to spend some money on it eventually.

If its the manifold gasket you should still replace that turbo.
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Re: Mysterious whistle under load

Postby geeves » Fri Mar 22, 2019 4:30 pm

I disagree.
Turbo requires replacement if there is any side play, more than 0.5mm end float or has a whistle like a jet winding down on shut off. Even this last one is debatable as some turbos have gone years with that noise (although some havnt gone weeks as well)
Otherwise your changing a good turbo. The new turbo I put on my wizard whined under boost from new till I sold it 2 years later and my current bighorn has the same whine from when I bought it 6 years ago till now and anyone that knows me will tell you Im not gentle with it
Sanding your knuckles before starting work can help. That way you cant skin them
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Re: Mysterious whistle under load

Postby ElIsuzu » Fri Mar 22, 2019 6:03 pm

Thanks for your input Geeves. Here's a video of the noise: https://www.youtube.com/watch?v=Cv4Z8iE ... e=youtu.be

So I've been looking into it further. It has had an EGR delete - the previous owner has removed the EGR, cut off the flange where it attaches to the exhaust manifold, welded a fat plug over the flange and reinstalled it - photo here: https://imgur.com/GIslOSa

I've made and installed a new blanking plate and sealed it with a liberal application of red gasket goo. There is no change in the nature of the noise at all!

It still might be something to do with the EGR delete though... It's also worth noting that the noise started occuring shortly after I replaced the alternator, but it occurs even when the alternator belt has been removed.
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Re: Mysterious whistle under load

Postby Sandpitmatt1 » Mon Jun 07, 2021 2:59 pm

Hi did you ever solve this? My bighorn has recently developed the same noise and I can’t put my finger on it either. I’ve still got my EGR and have also had a new alternator installed recently but doesn’t seem to be a belt noise.
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Re: Mysterious whistle under load

Postby geeves » Mon Jun 07, 2021 5:39 pm

Some turbos whistle. The genuine turbo I put on my Wizard whistled from new. The current bighorn has had the same whistle for the 6 years Ive owned it. Some people have even suggested the turbo is on the way out but it keeps chirbling on.
Only worry if
It whistles when you turn it off. This is an internal heat shield failure and the turbo will fail in the next 2 weeks to 5 years.
It screams loudly which could be bearings etc but other things like boost leaks or badly designed snorkles can cause this
Sanding your knuckles before starting work can help. That way you cant skin them
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