300Tdi Loss of Top End Power

Muddy31 · Mar 26, 2015

Hi,

My 1994 300Tdi 90 has lost it's top-end power. Struggles to reach 68-70mph on Mway unloaded, where could prev reach 85mph. Loses speed even on slight inclines. I know they're not built for speed but this is my indication that something's wrong.

Has recently had oil and coolant changes with no issues, as well as new oil, air and fuel filters. Oil, coolant and temp levels are all fine. Has had new lift pump.

Any ideas on most likely cause? Valve clearances? Head gasket? Intercooler? Intercooler pipes? Anything else?.... And in which order would you investigate?

Incidentally, this problem was only apparent after a gear- and transfer-box re-con. Could this be related?...

Dippypud · Mar 26, 2015

Had a new air filter ?

Checked for a split in the boost hose from the turbo to the FP ?

Any leaks from the newly fitted stuff ?

Muddy31 · Mar 26, 2015

Forgot to mention, it's registering 0.98-1.0 bar pressure on the boost gauge just before the actuator.

Muddy31 · Mar 26, 2015

Yeah, had new air filter and all new stuff seems fine. Not checked the boost hose yet.

vilguy · Mar 26, 2015

check the hose from the turbo to the fuel injection pump. If it has a pin hole leak it will be massively down on power.

Kwakerman · Mar 26, 2015

Did the replacement gearboxes have the same ratio's as the ones you removed? Also transfer box speedo gear may be the wrong colour (different tooth count) and make your speedo read differently than before.

Muddy31 · Mar 26, 2015

Gear- and transfer box work was allegedly recon of existing (not replacement) but I sometimes wonder what they did since part numbers don't match original (but they may not have when I bought it anyway, as I didn't check them then). Interesting comment on speedo gear, but it feels like speed is accurate. Maybe turbo hose is next thing to check then....

MeatCleaverX86 · Mar 27, 2015

I had similar issue years ago around town low speeds didn't notice any difference only in 5th was the fuel pump seized not giving extra fuel on boost 1 new fuel pump later perfect

Muddy31 · Apr 7, 2015

Hmmm, that makes for somber reading... :-(

My two latest quandries:

1) As above, if the problem is a seized fuel inlet pin within the fuel pump (ie. the fuel pin doesn't come out when the cone goes down), surely with some penetrating oil and persuasion the pin can be freed??

2) To test for a problem on the air feed line from the turbo to the fuel pump, and determine if this is affecting the fuel pump; if I disconnect this pipe (fuel pump will therefore only get atmospheric pressure), should I bung the turbo end of this pipe where it leaves the actuator?

Kwakerman · Apr 7, 2015

If you remove the banjo at the pump you could use a bolt, two washers and a nut through the banjo to seal it. That said, for a quick check leaving the pipe open shouldn't cause too much of a pressure leak.

boguing · Apr 7, 2015

Don't panic. The best way to find a fault is to be methodical. Try Dippypud and Vilguy's suggestions first.

Muddy31 · Apr 7, 2015

Thanks Kwakerman.

Tried it this evening simply with the actuator-to-fuel-pump hose disconnected. There's a big difference (much less power), as hoped, so I think the hose is working fine (gets 0.99bar at full boost when reconnected).

Back to considering dodgy air pipes / condenser, or potentially seized full pump.... Can anyone help on previous point (1) above?

Muddy31 · Apr 7, 2015

Thanks Boguing....apologies I overlooked you!

As above, gonna go with Vilguy and Dippypud's suggestions next, ie check air pipes, condensor, and then fuel pump (if I'm not already fixed by then and out racing ferrari's)....

Kwakerman · Apr 7, 2015

Muddy31 said:
Thanks Kwakerman.

Tried it this evening simply with the actuator-to-fuel-pump hose disconnected. There's a big difference (much less power), as hoped, so I think the hose is working fine (gets 0.99bar at full boost when reconnected).

Back to considering dodgy air pipes / condenser, or potentially seized full pump.... Can anyone help on previous point (1) above?

I think you have effectively checked the fuel pin, with the pipe disconnected the pump diaphragm wont have been pushed down by boost pressure, therefore the fuel pin can't move out into the cone recess and will not increase the fuelling (therefore less power). With the pipe back on the diaphragm will have moved down letting the pin out and more fuel to engine hence more power.

Muddy31 · Apr 21, 2015

Done more tests and would appreciate your thoughts

With the actuator-to-fuel-pump hose disconnected I still get a reduction in power when I push beyond about 95% throttle, in 4th/5th gear. I can hear and feel what seems like a slight loss in boost, and theres a drop of about 0.1bar on the boost gauge at this point. If I ease back off to that same spot, increased boost resumes. If I stay at that spot, speed increases slightly, but anything beyond that point, speed levels out.

I think this is either a pinhole leak in a flexible pipe (but I cant find anything wrong in the pipes), or the turbo? Theres only about 0.5mm play on the impeller axle but Ive noticed some oil in the mouth of the air supply pipe and the actuator-to-fuel-pump pipe. Is this indicative of a problem? The turbo-to-condenser pipe is dry.

If it is the turbo how would you go about checking it? Seals? Is a full recon necessary?

cborrman · Aug 16, 2020

isnth

Dippypud said:
Had a new air filter ?

Checked for a split in the boost hose from the turbo to the FP ?

Any leaks from the newly fitted stuff ?

is that this hose? I lost power on a drive and this looks decidedly squidgy

Al2O3 · Aug 16, 2020

cborrman said:
isnth
is that this hose? I lost power on a drive and this looks decidedly squidgy

No, that's your rocker cover vent pipe.

Dippypud · Aug 17, 2020

cborrman said:
isnth
is that this hose? I lost power on a drive and this looks decidedly squidgy

No it's the little thin one and 'squidgy' is a technical engineering term not belonging to that hose ...

It'll have a split, tear, hole or just have fell off of it's perch ...

cborrman · Aug 18, 2020

Dippypud said:
No it's the little thin one and 'squidgy' is a technical engineering term not belonging to that hose ...

It'll have a split, tear, hole or just have fell off of it's perch ...

thanks for help both, it had fallen off its perch thankfully, and you are right, it was purposefully pert, not at all squidgy... and had squirted oil... ok, will stop now

300Tdi Loss of Top End Power

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