Altitude issues

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Altitude issues

Postby Ratty 63 » Thu Apr 06, 2017 9:55 pm

Hi all,

Been a while since I have ventured here .... everything had been working as it should so no need!

This may have been answered elsewhere so please point me in the right direction if it has....

After a recent run from our normal stomping grounds around the Gold Coast and Brisbane to Toowoomba we discovered that the engine runs lean when at higher altitudes. Initially we thought is may have been fuel quality or a heat issue after climbing the range into Toowoomba but it remained lean until we came back down...

We have since had the same problem heading to Warwick (up the range again) and just last weekend we drove up the hill to Tamborine Mountain and had the same problem. I can tune it out by changing all the settings in the fuel map but this is clearly not the correct way to deal with this as it will then run rich at (closer to) sea level.

Can anyone shed any light on this? Are there any variables that have an effect on the fuel at higher altitudes and if so, what it the best way to configure them?

I am happy to experiment/try anything to fix this as we are trying to get the car ready to go to Cape York in a couple of months and would like to spend some time in the hills behind Cairns along the way.
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Re: Altitude issues

Postby stormtrooper » Mon Apr 10, 2017 5:29 pm

This is quite a technical issue in that there are various "laws" of nature that indicate the higher you go from sea level the thinner the air is. The thinner the air is the less fuel you need because the engine only needs to run at stoich (14.7-1 for regular petrol) at 85-90% of the fuel curve. If you have less oxygen you obviously don't need as much fuel but it should still run at the same AFRs that you set before. How do you know it is running lean?

This is all tied in with barometric pressure. I haven't got my Kalmaker laptop with me atm so I can't check what the relevant variable is but I'd be looking at something with baro or bar in it . Having said all that I come back to the question of how do you know it is running lean?

How are your O2 sensors?
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Re: Altitude issues

Postby Ratty 63 » Sun Apr 16, 2017 2:28 pm

Hi Stormtrooper,

Initially we suspected it was lean at higher altitudes as it began to pop-back through the intake at anything except WOT (which we have set to be fairly rich). The O2 sensor gives some very strange readings on this vehicle (it was replaced with a new one just recently with no change - but this is another issue for later). We are running it in open loop mode, so presumably the lack of O2 sensor shouldn't effect us (correct me if I am wrong!). Anyway, to confirm what we thought we have borrowed a stand-alone air/fuel meter and whilst it reads in the "normal" range at sea level, it shows a lean mixture when up a hill. I can stop the backfiring by pushing up the fuel levels temporarily at the current point in the fuel map - but this leads to a super rich state when we return to sea level if left in this state.

The problem seams to come on suddenly when climbing a hill - its fine up to a certain point in the climb then suddenly lean... and vice-versa on the way down.

We have noticed during tests in the last couple of days that should the BAP reading drop below 100kpa problems begin - I have just been combing through the variables and have found a couple of variables which, from their description, would appear to have something to do with the BAP (VAR:BAP:DEFBAP is most noticeable as it is currently set to 100kpa). Should I be dropping this value to, say, 80kpa? Am I on the right track?
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Re: Altitude issues

Postby stormtrooper » Sun Apr 16, 2017 4:57 pm

You run a VW Beetle don't you? How is it setup?

Ratty 63 wrote:Initially we suspected it was lean at higher altitudes as it began to pop-back through the intake at anything except WOT (which we have set to be fairly rich).
The O2 sensor gives some very strange readings on this vehicle (it was replaced with a new one just recently with no change - but this is another issue for later). We are running it in open loop mode, so presumably the lack of O2 sensor shouldn't effect us (correct me if I am wrong!). Anyway, to confirm what we thought we have borrowed a stand-alone air/fuel meter and whilst it reads in the "normal" range at sea level, it shows a lean mixture when up a hill. I can stop the backfiring by pushing up the fuel levels temporarily at the current point in the fuel map - but this leads to a super rich state when we return to sea level if left in this state.
You've got to be very lean to get backfire because of AFR. What you describe could be caused by any number of things so a bit of diagnostic work may be required. I, personally, would run in closed loop up until 85-90 kpa and let the O2 sensor help control the fuel more.

Ratty 63 wrote:The problem seams to come on suddenly when climbing a hill - its fine up to a certain point in the climb then suddenly lean... and vice-versa on the way down.
Have you checked the MAP sensor?

Ratty 63 wrote:We have noticed during tests in the last couple of days that should the BAP reading drop below 100kpa problems begin - I have just been combing through the variables and have found a couple of variables which, from their description, would appear to have something to do with the BAP (VAR:BAP:DEFBAP is most noticeable as it is currently set to 100kpa). Should I be dropping this value to, say, 80kpa? Am I on the right track?
I wouldn't be dropping anything yet, what I would do is make sure your sensors are working as they should.

If this was a car that come into a workshop I'd follow this process before I did anything else.
1. Check ECM for fault codes. Run through the manufacturers fault finding process to fix any codes that are displayed
2. Check obvious servicable items, e.g. air and fuel filters, coil(s), spark plugs, and leads. (I'd do this because electrical issues that cause backfires can make O2 readings lean because of the extra oxygen in the exhaust).
3. Verify AFRs.
4. Check fuel pressure.
5. Verify AFRs.
6. Check sensors, e.g. MAP, Engine Temp, Intake Air Temp. All 3 of these along with O2 sensors affect AFRs.
7. Verify AFRs.
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Re: Altitude issues

Postby Ratty 63 » Sat Apr 22, 2017 1:23 pm

stormtrooper wrote:You run a VW Beetle don't you? How is it setup?


Yes - well kind of a Beetle anyway! It's a Baja (off road (but road registered) modified Beetle (cut body, shortened guards).
The engine was designed and built by my mechanic brother (who owns the car) and is based on a VW Type IV 2 litre engine which is usually found in a mid-to-late 70's Kombi, but this one has been upgraded to 2.5 litres (standard stroke, bigger bore and a custom Porsche style cooling fan). It runs a monster cam and massive valves (for a VW). There are two dual 52mm throttle bodies sitting on top of very short manifolds - these have a balance pipe between them which is where the MAP sensor is connected (and a take off point for the vac operated diff lock - another story). The MAP sensor is mounted on the firewall in the engine bay. The IAC valve is connected to a hose that runs between both throttle bodies and takes its air from one of the air filters mounted on top of the throttle bodies. Due to the huge cam overlap, very short manifolds and massive throttle bodies it has a rather stunted vacuum pattern. The engine runs between 40kpa (off throttle at revs) and 100kpa. Normal run (cruise) at 100km/h is around 80kpa.
Spark control is handled by a Camira distributor which has had the original mounting flange machined off (they fit straight into the VW crankcase without any further mods once you remove the flange). Coil is stock Camira and is also mounted on the firewall.
The TPS is a stock unit from a HSV Commodore and is attached to the LH Throttle body. The MAT sensor (stock Camira) is in the manifold on the same side. We are taking the engine temp reading from the LH head (as the LH head normally runs hotter - it a VW thing!) using a VW aircooled EFI head temp sensor which goes though a custom circuit that feeds the ECU a reading that makes it look like an comparable water temp. The reading shown at normal operating temp is around 82-85 deg but we do see it climb into the low 90's when pushed hard or leaned out.
The 3 wire O2 sensor is mounted in the extractor system where the four pipes merge into one - as I mentioned in one of my last posts, it is currently disabled as we are not getting sensible readings from it....
There is no speed sensor connected... yet.
The vehicle is fitted with air-conditioning - not relevant for this conversation....
I am aware that an engine with these specs, set-up in this way, would be better suited to a dirt track race car than something used on the road or at low revs crawling around off road.....but my brother doesn't agree - so we battle on with tuning the angry pig!

stormtrooper wrote:If this was a car that come into a workshop I'd follow this process before I did anything else.
1. Check ECM for fault codes. Run through the manufacturers fault finding process to fix any codes that are displayed


Checked - no fault codes found.

stormtrooper wrote:2. Check obvious servicable items, e.g. air and fuel filters, coil(s), spark plugs, and leads. (I'd do this because electrical issues that cause backfires can make O2 readings lean because of the extra oxygen in the exhaust).


Checked - This engine has only just been rebuilt with a slightly less aggressive cam (not 'less-aggressive' enough for an off road car in my opinion) and all serviceable items, including the leads were replaced with new items. We have even put the old leads back on just to check, but no change. This altitude problem was an issue before the rebuild too.

stormtrooper wrote:3. Verify AFRs.


Re-checked using a stand alone AF meter at sea-level - all looks OK throughout the range.

stormtrooper wrote:4. Check fuel pressure.


Checked - within normal range with normal variance under load/low vacuum.

stormtrooper wrote:5. Verify AFRs.


No setting changes made from point 3.

stormtrooper wrote:6. Check sensors, e.g. MAP, Engine Temp, Intake Air Temp. All 3 of these along with O2 sensors affect AFRs.


Last weekend we changed all the sensors to the brand new spares that are normally carried in the car (This car can be days away from a parts shop during some trips). No change to any of the readings or behaviour observed. Still have odd readings from the O2 sensor but for the time being we are running in open loop mode. All other readings from the other sensors appear as I would expect to see them.

I hope that my last post made it clear that the issue is not just occurring when climbing to higher altitudes but is always present when above sea level. Even if the car is left to cool-down to stone cold state whilst at higher than sea level it still has lean-run issues until it is returned to sea level.

During last weekend we took it for a run up Tamborine again and fiddled with every variable that I could find that may have had an effect. I changed the VAR:BAP:DEFBAP to 80 and found that it had an effect but not enough to resolve the problem. I also changed all the associated variables and found the same thing - minimal effect. I dropped 3-5 degrees from the spark advance - this stopped most of the backfiring but according to the AF meter it was still running lean. This also made the engine noticeably less responsive when we returned to sea-level (as you would expect). I spent time looking for air pressure related variables, both spark and fuel, not sure if I found them all, but I was unable to find anything that had any real effect without really messing with the general running/smoothness of the engine. I was only playing with one variable at a time so perhaps using them in combination in some way might sort out the problem....?

Over to you - any suggestions or ideas?

I'm happy to try any suggestions, take readings and report back finding if it will help.

Thanks (and sorry about the length of this post!)

R :)
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Re: Altitude issues

Postby stormtrooper » Sat Apr 22, 2017 3:29 pm

I'm going to have to crank up my old Kalmaker laptop to have a look at the variables again. In the meantime could you attach your bin and I'll have a look at that to compare it to other 4 cylinder bins.
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