Help me AEU86, you're my only hope

[Nakilad]

Morning Everyone,

This might be a bit of a rant but please bear with me.

First the question: Can an Engine internal mechanical issue (bottom end) lead to constantly lean fuel mixture?

The context:

The engine had a rebuild around three years ago. Since then it has lacked top end performance. It seems to all intent and purposes that it is starving of fuel above 4000 ish rpm.

I have recently installed an exhaust manifold with 02 sensor port, and installed a narrow band 02 sensor, which gives me around 0.14 volts, no matter where I position the mixture screw.

And what’s been checked / replaced:

Mechanical Parts that have been swapped out for at least one other include: Throttle body, Inlet Manifold, TVIS, Fuel Rail, Fuel Filter, Fuel Pump and even Cylinder Head.

Electrical Parts that have been swapped out for at least one other include: Engine Loom, ECU, TPS, Air Temp Sensor, Water Temp Sensor, Injectors.

Swapping these components has made no noticeable difference to the performance of the car.

Other Checks:

Electrical: voltages regarding temp sensors were checked at the ECU and were between 0 – 5 volts as expected. Disconnecting the TPS or MAP sensor causes engine stall. Disconnecting temp sensors (water or air) makes no discernible difference to O2 readings.

Air Leaks – Hand over throttle body causes immediate engine stall. Blocking of brake booster line has no effect on engine. Blowback from inlet cam cover does not seem excessive, and does not stall the engine when blocked, but does increase O2 readings for a short time if blocked for 30 seconds and then unblocked.

Fuel – pressure and leakdown tests all ok. Individual flow test not done, but injectors were cleaned before the engine rebuild and worked fine then. Second set of injectors (which actually do leak a bit) had no noticeable effect on engine performance or o2 readings.

Compression test was done last year, don’t have the figures to hand but they were all within spec and tolerance. Cam timings have been checked and are aligned correctly. Block was very lightly skimmed before rebuild if I remember correctly.

Engine starts pretty easily, revs ok when not being drive, but idle has been difficult to set (probably related to lean burning).

I feel like I’ve replaced pretty much everything except the bottom end. Crazy as it sounds I am wondering if there could be something there, even though that make no sense to me!

Any ideas, as random as they might be, would be appreciated!

[The Doctor]

Maybe another stupid idea, you don't have a partially blocked exhaust? If the engine can't get rid of the exhaust gasses quick enough it also might underperform and do weird things.

Like you say, bottom end shouldn't be the cause of the problem.
And i don't really know about the O2 sensor thing, what is the value you should see normally?

[Nakilad]

(03-14-2021 02:53 PM)The Doctor Wrote:  Maybe another stupid idea, you don't have a partially blocked exhaust? If the engine can't get rid of the exhaust gasses quick enough it also might underperform and do weird things.

Like you say, bottom end shouldn't be the cause of the problem.
And i don't really know about the O2 sensor thing, what is the value you should see normally?

After 18 months or so of trying to sort this thing out, there is no such thing as a stupid idea anymore!

I put an HKS Legal Catback on it back in 2019, and the manifold has been on and off a few times recently so I don't think there is any problems there. The center muffler however, has not been replaced, so there might be an issue there. Its worth having a look at!

The narrow band O2 sensor should output around 0.5 volts, for a 14 to 1 ratio. Anything under that is lean and anything above is rich. Its not as accurate as a wide band of course, but its enough to at least get an idea of whats going on.

[The Doctor]

Just something what came to my mind...

Similar problems on a mates volvo. Gradually got worse and worse untill it wasn't driveable anymore.
Looked like fuel pressure issues. Replaced everything, but no solution.

In the end the little hose which came off the fuel pump to the hard line outside the tank had a little split in it. So it would leak fuel back in the tank whenever the pump was making pressure. If the o2 sensor is reading leab, it must be something fuel related.

We measured fuel pressure multiple times and it all seemed OK (untill the end when the hose finally totally failed). But when the engine needed more fuel on the road, it just couldn't deliver it due to the split in the hose. When we took it out, it wasn't clear, but after further investigation we found the split in the hose.

[jondee86]

Something to check... when the engine is idling, if you pull the vacuum hose off
the MAP sensor the mixture should go very rich. If nothing changes you need to
check the MAP sensor by applying a vacuum to the sensor and seeing how the
output voltage changes. The testing procedure is outlined in the FSM.

Cheers... jondee86

[Nakilad]

(03-14-2021 09:32 PM)The Doctor Wrote:  Just something what came to my mind...

Similar problems on a mates volvo. Gradually got worse and worse untill it wasn't driveable anymore.
Looked like fuel pressure issues. Replaced everything, but no solution.

In the end the little hose which came off the fuel pump to the hard line outside the tank had a little split in it. So it would leak fuel back in the tank whenever the pump was making pressure. If the o2 sensor is reading leab, it must be something fuel related.

We measured fuel pressure multiple times and it all seemed OK (untill the end when the hose finally totally failed). But when the engine needed more fuel on the road, it just couldn't deliver it due to the split in the hose. When we took it out, it wasn't clear, but after further investigation we found the split in the hose.

I replaced the tank at the same time as the engine was rebuilt, replaced the internal line from pump to the housing like you described. Also, put an OEM pump in last year, just to make sure that wasn't an issue either. No change that I could tell.

I agree that it has to be fuel, somehow, but where? Fuel pressure jumps when blipping the throttle as per the Service Manual, also disconnecting vacuum line from the fuel regulator causes the pressure to rise, but funnily enough doesn't change to O2 readings. Its very odd.

Appreciate the idea though, keep em coming!

---

(03-14-2021 11:36 PM)jondee86 Wrote:  Something to check... when the engine is idling, if you pull the vacuum hose off
the MAP sensor the mixture should go very rich. If nothing changes you need to
check the MAP sensor by applying a vacuum to the sensor and seeing how the
output voltage changes. The testing procedure is outlined in the FSM.

Cheers... jondee86

I've tried 3 different MAP sensors, no noticeable change. Disconnecting the MAP sensors causes the engine to stall, but I'm not sure what just dropping the vacuum line off will do, I'll try that tonight or tomorrow. Thanks for the idea.

[RSR-K]

Den Unterdruckschlauch vom MAP-Sensor abziehen und nicht den Stecker. Dann müßte er anfetten.

[totta Crolla]

Is the O2 sensor feeding back to the ecu or are you just monitoring the output? Dependant on its location, If the sensor is un-heated it may never give a true reading. Check and double check the ignition timing and if necessary turn the distributor to see if things improve. Be aware of engine knock when doing this.

[jondee86]

There is another thing that you can check. If there is a partial blockage (crimped pipe etc)
in the fuel delivery you could be getting enough fuel to maintain engine performance at low
rpm, but not enough to support high rpm/load operation. If you can get a test gauge (or find
a garage that has one) you can monitor the rail pressure at idle and see what happens when
you rev the engine.

If the pressure drops substantially when you snap the throttle open, you have a restriction
somewhere. Pressure at idle should be in the range 30-33psi and if you disconnect the FPR
vacuum compensation hose from the FPR the idle pressure should rise to 38-44psi.

Cheers... jondee86

[Nakilad]

So, i was finally able to get back to the car today for a few hours. I'll start by answering the last two comments:

@the Doctor, I dropped the exhaust off at the end of the exhaust manifold and started the engine. Made no difference that I could tell, apart from being quite a bit louder!

@rsr-k No difference, check the videos though underneath though,

@totta Crolla, The O2 sensor is just plugged into a Prosport Gauge. Single wire narrow band sensor, mounted in the exhaust manifold as seen in the later models. It is therefor unheated. I have tried 2 distributors, and moved the timing from between 10 and 15 degrees (static) with no noticeable effect.

@jondee86, I have a pressure gauge mounted to the fuel rail at the moment. Fuel pressure is perfect at idle (2.2 bar (32 psi) from memory) and jumps to close to 3 (43 psi) when blipping the throttle from idle.

So, I was unable to really check the voltages from the MAP sensor as jondee86 was mentioning, ran a bit out if time and don't have a mightvac handy to verify the vacuum to voltage.

I did however find something interesting/ strange. I have been measuring the output of the O2 sensor with a digital multi-meter with the +ve attached to the wire of the O2 sensor and the -ve attached to the battery -ve. This showed 0.14 volts and the gauge registered as lean. Then for whatever reason I moved the -ve of the multi-meter to the +ve terminal of the battery. This gave a reading of -14 volts (as you would expect), but additionally, the gauge now show the mixture in the optimal range! Something is still not right however as in this condition, blipping the throttle causes the gauge to register lean rather than rich.

To help with this, I made a few short videos of the various conditons and uploaded them to my youtube channel (Playlist here). Maybe something in there will ring a bell?

If I can get the car running normally at idle I would be happy to go for a quick squirt up the road, but as it's not had a TÜV for almost a year now I don't want to do that too much!

As always, I appreciate all the ideas, thanks for trying to help me out here.