I drove the 617A motor a lot over the last few days. 1100 miles overall!

I posted before how I don't seem to have the power I used to have before all the big problems I had last year when I had to replace my inj. pump... except that power seemed to return to some degree on this trip, only it was on and off.

I would notice this odd bump in power sometimes and it was in the mid-upper RPM range when I was accelerating. It felt like another cylinder would suddenly start working... or as if a car had pushed on my bumper from behind. It was a good 15% bump in power and it was instant, not like a turbo bump which is smoother and takes a couple seconds. This was OFF and then ON. I have a boost gauge so I can watch the pressure and nothing was weird there. Didn't notice anything odd about the exhaust smoke (I barely have any no matter what I do)

TONS of recent service from my big break-down problem last year...
Fuel filters are basically new
Valves adjusted a week ago but were 95% in spec anyway
Diesel purge done recently
Was running my usual diesel additive
Throttle linkage checked
No leaks anywhere
New fuel lines and lift pump from last year
Just replaced manifold gasket
Turbo in exc. shape, had about 10lbs of pressure at max throttle
ALDA backed all the way out
New air filter

Out of ideas... other than maybe an injector has some weird issue? I doubt that though. Could the timing somehow change slightly, suddenly?

Just a thought, but...
Have you cleaned your fuel tank strainer lately(ever)?

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97 e300d, 78 300Dt, 95 E300d, 94 E320 estate

Maybe go for a drive without the fuel filler cap in place?

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD

Tried the cap and no difference. The tank is new with new lines so I really doubt the pick-up.

Also, this is pretty consistent and doesnt feel like a fuel supply issue, at least not the supply before the pump.

I was able to better feel it out today. I would go into 2nd gear, for example, and would watch the boost gauge. I would rev up fairly well into mid range to higher range revs, and would hold the revs constant and be at about 4 PSI for 5-10 seconds, then I'd feel the burst of power and my pressure would go up to 6-8 PSI, but I think the pressure increase is due to the engine revving up/getting more fuel rather than it being a turbo related issue (that's my feeling about it anyway). I did just inspect the turbo completely and nothing seems out of place or odd.

So it feel like fuel is being held back until higher revs are held for a little while. It will repeat this again in the next gear, usually, in the same way. Weird!

My first check would be the manifold vacuum/boost to ALDA line. No matter what your manifold boost pressure is, if the boost signal isn't getting to the ALDA then it can't give you any more fuel, and a clogged line and manifold port is a common issue from all the EGR junk.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

It's the opposite, really. The ALDA keeps fuel in check until there's boost and then it basically does nothing. Correct about the smoke, it's just off-boost that it regulates it.

Also, I don't immediately think it's an ALDA problem either. I've had all manner of configurations with the ALDA, including none, and never experienced that. I could maybe see it if it were getting hung up in the fuel-limiting function and then suddenly popping at a certain point to let more fuel in. You'd have to take it off and really go over it to check though. Meanwhile, with it off you could drive it and see if it still does that. Not a bad idea, actually.

-Rog

Drove it tonight and took careful notice of this issue:

I could rev up, say in 2nd gear, and all I need to do to get the power boost is hold the throttle in the mid RPM range for about 5-7 seconds, and this leads to the power bump.

How much the turbo's pressure was reading didn't seem to matter... it's just when the throttle is consistently held for some time. When I go to the next gear, the process starts over.

I also noticed that my take off from a stop seems slower so the power boost could be more accurately described as a brief return to a normal power level and this lasts until the throttle is released. (So gear changes and stops "resets" the issue)


I'll try taking the line off the ALDA to see if there's a change but I think this is something else.

You don't say what car this is, so I'll have to guess its similar to my '83..

If you're getting boost but no black smoke before the surge, especially with the ALDA backed all the way out, then its gotta be fuel limited. Filters, lift pump, pressure regulator, something.

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

I agree, but what makes sense here?

What would hold fuel flow for a while, then release it after revving for a short while? I can't get my head around this one yet.

It sounds like one of your filters has a leak. As you hold the throttle under firm pressure, something fills up the gap and then the fuel flows in full stream. Except that you would certainly know if fuel was leaking underneath. Or one of your filters has an obstruction that only moves out of the way after you hold the throttle firm for five seconds.

Does this same analogy work for an air leak?

I've checked and rechecked for leaks and its all bone dry and air-tight. Especially after the IP failure I had last year which was a very long post here.

I replaced all the fuel lines, washers, etc.. I have an almost all new fuel system from top to bottom and all was fine until recently when these power bumps started.

It still sounds like intermittent switchover valve or alda problem. Have you replaced the $25 switchover valve? Are the lines clear?

Remind me about the valve... are you talking about the bolt/spring on the side of the IP? Having a brain fart.

The lines can't be more clean and clear. All new. Filters newer and clear prefilter is spotless inside.

Is there some little filter screen somewhere Im forgetting?

The switchover valve is on the firewall. It gets a branch of the line from the intake manifold to the ALDA as well as I think two vacuum lines.

It would 100% point you in a productive direction if you just removed the ALDA and see if it still does that. You don't even have to take it all the way out, just zip tie it out of the way once you unscrew it. If it still does it then you can forget the ALDA and switchover valve. If not then it's something in that system.

-Rog