Bad running after new head gasket - cause?

[rogerthechorister]

A thought occurred to me today (as a result of an EGR pipe problem on my other Sport). The EGR valve opens at about 3,000 rpm doesn't it? Which is why it usually does not affect the MoT emissions. Well, what if there were a crack in the EGR body? Or a big hole in the EGR pipe (one of the two - one goes from the exhaust manifold to the back of the EGR valve and the other goes from the top of the EGR valve to the inlet manifold) that comes from the exhaust manifold. The mixture would suddenly go weak when the EGR valve opened, wouldn't it?

Against this theory, there would be a ticking or blowing noise as exhaust manifold pressure vented through the crack or hole.

Views?

[rogerthechorister]

Max the mecco says he has tried blanking off the EGR pipe at the exhaust end and it made no difference. But it wouldn't if there was a break in the first pipe section or in the EGR valve itself, would it?

Tuesday she goes to Tim the other mecco to try blanking the EGR off completely.

The other funny thing is that Max has been talking to the people in Essex who drift Volvos and they say that there should be a steady voltage on pin 24 of the ECU (which if my memory serves me is the 5 degree retard pin) and we do not have that. I cannot now remember whether we have 12V or zero V (I think it's 12 but I'm not sure - and at about 3,000 or 3,500 rpm it goes to a wobbly 3.5 volts. That MIGHT be a cracked wire in the loom. I don't think it's the crank position sensor as I'm pretty sure we've changed that and if it was the sensor signal then the reve counter would go funny.

Some optimism, maybe!

[Laird Scooby]

If pin 5 should go to 5V and it normally has 0V but at a certain point changes to a wobbly 3.5V then it suggests whatever is meant to switch that input either has broken wiring, dirty connections or the switch/sensor is worn out.

However as this problem occurred after having the head gasket done, i'd be inclined to think this wire got trapped between two metal parts as they were refitted and the insulation has been crushed, allowing some power through but not all of it. If it was 12V and this happened then it would probably blow a fuse somewhere.

Usually with inputs that are meant to have a definite voltage on them under certain conditions they work on a "complete change of state" basis, in other words either on or off, not dropping from a higher voltage to a fixed lower voltage.
I'd be looking for the sensor that is meant to feed pin 5 with 5V and work back along its wiring to check it's free to move and hasn't been squashed between (for example) the inlet manifold and the head.

Many moons ago a friend bought a Vauxhall Senator 3.0 24v for pennies (he got two for about £4-500, one MoT'd but not running, the other a spares car only) because it wouldn't start after having had the head gasket done. The fuel pump feed wire had been trapped between the inlet tracts and the head resulting in a burned out fuel pump relay and a blown fuse.
One fuse, fuel pump relay and wiring repair later and he had a highly tuned 24v Senator with no sunroof, wind up windows and cloth upholstery! Oh yeah, it was white as well with some funny zips in the headlining!

[rogerthechorister]

I'm pretty sure it's pin 24 - which I think is the pin to put volts on (or is it take down to earth) to retard the ignition 5 degrees.

[Laird Scooby]

Originally Posted by rogerthechorister

I'm pretty sure it's pin 24 - which I think is the pin to put volts on (or is it take down to earth) to retard the ignition 5 degrees.

Sorry i read it and somewhere between brain and fingers it changed to pin 5 - not sure what happened but i meant to type pin 24. If it's meant to go to 5v when boost reaches a certain level then retard the timing and it isn't happening then there's a good chance the knock sensor is sensing detonation through the timing being over-advanced because of the boost. If the knock sensor is being triggered that would retard the timing quite a bit i would think.

Not sure if you'd get away with simply unpluggng the knock sensor to test the theory or if you have a spare one cable tie it somewhere it won't get knocked and plug that one in for a "once only" test to prove the point - just don't go too mad while it's over-advancing with boost or there could be other problems as a result but it should tell you if you're on the right track.

[aardvarkash10]

Originally Posted by rogerthechorister

...I cannot now remember whether we have 12V or zero V (I think it's 12 but I'm not sure - and at about 3,000 or 3,500 rpm it goes to a wobbly 3.5 volts. That MIGHT be a cracked wire in the loom....

Some optimism, maybe!

Definitely optimism.

The output of the ECU is digital at ttl (transistor/transistor logic) so should be either 5v or 0v (actually around 0.8v). A floating voltage can indicate a fault in the ECU (essentially a transistor is not switching fully "on" or "off") OR that the meter being used is not fast enough to read the signal and is "averaging" a pulsed signal that is swinging from 5v to 0v rapidly. I'm picking the latter.

an odd ttl output reading (like 3.5v) cannot be an input sensor problem.

As its a voltage signal and effectively has no current, a partial wire fault is very unlikely to cause an error - the current is too low to create a voltage drop and hte singal will still (or should still) be either logic high or low, 5 or 0v.

Realistically, the best way to test the output of a logic circuit like this is with an oscilloscope, not a voltmeter. I am guessing here, but I think you will see a high frequency squarewave output swinging from 5v to 0.8v and rms averaging around 3.5v

[rogerthechorister]

It cannot be an ECU fault. Three different ECUS are all perfect in my other Sport and duff in this one. And the other point appears to be that pin 24 is an input to the ECU not an output from it. As far as I know. But thank you for the suggestion.

[Laird Scooby]

I've had a good scour of t'internet and i'm guessing your ECU is the one ending in 937 for the B230FT?

The only pinouts i can find are for the B230F (and not the 0 280 000 937 ECU) and it shows pin 24 as an input but only 0 to 1V as it's the signal input from the Lambda sensor.
If i could find the pinouts for the 937 there's a fair chance pin 24 would still be an input and probably a logic level (TTL if you will) 5v input in which case my original theory could still apply.

Ashs idea about a pulsed output signal could still be but knowing Bosch and how they like to keep certain pins as inputs and certain pins as outputs it's a fair bet it is an input, although not from the Lambda sensor!

They're not going to change a whole circuit board just for adding a turbo so altering the input by changing the software and perhaps adding a resistor to drop the voltage or removing a link so the variable voltage from the Lambda goes through a DAC first.............

[aardvarkash10]

if its input from the lambda, it can only swing 1v, not between 5 and 0

http://ipdown.net/jetronic.info/tiki...nector_pin_out

[rogerthechorister]

I'll check the ECU number in the morning if I can find the box of bits that has been changed. 937 sounds familiar though. However - - - she had a new lambda sensor during one of the very first attempts (after the head gasket) to cure the misfire. I remember the rueful look on the face of John the garage proprietor - who was a staunch methodist and would never lie or mislead - as he said "When I looked at the state of the old lambda sensor I really thought I'd solved it, but then I put a new one in and it wasn't that".