So lately as I've been driving down the highway, I will hear small pops from the engine, as if air is forcing itself in. Is this normal on older diesels?

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- 1979 W123 240D 4-speed, 390k miles

No, 99.999% sure that's not possible.. though id actually have to hear and see it for myself..... After accelerating over idle speed , mine only "RoOR".. What your describing sounds exactly like what it is, air getting into the lines with increased fuel flow.

When's the last time you replaced the injector return hose(s)? I'd start there (cheap and easy)... don't pry on the injector nipples when trying to get the old stuff off.. If you scratch/ deform those nipples in anyway, you can almost guarantee you'll have a fuel leak / trashed injector.

sounds dumb but, you replaced both fuel filters, yea?

Second, I would check to make sure that your manual primmer pump is screwed down into the lock position (assuming you have the older style primer pimp). Also, try seeing if you can 'prime' the system using the manual pump.. If fuel starts spraying all over then getting a new one (around $20-$30) is in order.

Make sure your cigar hose isn;t leaking or there isnt any obvious fuel leaks.

my last guess would be that your Injectors are worn out (when where they last replaced?) , spraying poorly and thus, causing the knocking noise your hearing.

Anyway, sure others will chime in but, if its loud enough for you to notice it then perhaps something 'just ain't right'

Goood Luck!

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1983 Euro 240D 277K
2016 Mazda 6
Coming Soon, 2013 Mercedes E350 Bluetec
Formers:
2012- Mazda 3
2007 Outlander- complete pile
1995 E300 Diesel 208K
2007 VW Passat 2.0T 63K SOLD
2003 Jaguar X-type 3.0 Sport Sold 04/11 w/88K
2009 Lexus RX350 9061 miles. Sold 04/09
2008 Mitsubishi Lancer GTS:Totaled Sept.,09 @ 24,000
2003 Infiniti I35 58K Sold
1984 300 turbo diesel 222K

I think you need to try to describe the sound a bit better, be clear on how loud it is, does it happen in certain circumstances, etc. Can you 'feel' something happen when the sound occurs, or is it just a noise?

Basic question: has the car had regular fuel filter changes and valve adjustments?

And are you sure this is an engine sound and not something from your suspension. A 'pop' while driving on the highway sounds more like trim, suspension or even driveline issue, than an engine issue.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

Thanks for the input, fellas!

It's just an audible thing. I haven't replaced the injector/fuel lines or the filters quite yet. I'll look at the records to see when they were last replaced. I'm pretty sure these are the stock injectors. To be more specific, it's making that sound when I accelerate at highway speeds. The popping is light but sporadic; the loudness depends on how much he engine is accelerating. Haven't taken it on the freeway, yet, but it's barely audible at a 40-50 MPH cruising speed.

I was told when I got the car by a friend to look into replacing the fuel lines and filters in the near future, and to look into new injectors. I guess now's a good time to start!

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- 1979 W123 240D 4-speed, 390k miles

I think that a video is in order, if you can get one safely.

Sincerely,

PE

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83 240D - 4 speed manual - Manilla Beige
189K miles, Tachometer mod, cool wooden shift knob from PeachParts, CocoMats, Original factory paint, manual windows, manual sunroof. Starting to add AudioWrap to this car too!