84' 944na - Spark, Fuel, Correct timing - NO START :-(

[cabbage]

Ahoy Gents,

I've been stuck on a 944 for weeks now and am open to any suggestions. Car was running well in the winter, but stored until a few months ago when it joined me

Background

When it got to me the fuel pump had died as the fuel tank had run dry and the pump sized. It also had some electrical gremlins caused by damp under where all the positive cables are collected behind the smaller fuse box causing a no crank situation - this was resolved and all was well

Cam tower was swapped as my friend supplied a painted item - this new tower was as far as i could see exactly the same as the old one, as i compared them side by side, testing how far the lifters move and when based on timing - they seemed exactly the same to me

current situation

when i crank, it tries to start, firing on the first and maybe the following cylinder per cycle but no start

i do get a random pop out the exhaust every now and then - but i think this is unburnt fuel reaching the exhaust


What i've tested / checked

1) I've replaced the fuel pump and confirmed that fuel gets to the rail and all injectors function correctly and spray.
2) spark is good and gets to all plugs
3) timing is correct, and rotor arm is in correct location when tdc compared to my other 944's
4) air flow meter tested and returns correct values
5) crank position sensors seem correct resistance - i also swapped with good items just to make sure and no change
6) throttle position sensor and the closed switch work correctly and throttle is setup correctly
7) compression is ok - a little weak on two cylinders but it seems unlikely this has become an issue as the car started and ran fine with compression as is - but maybe im wrong
earths are good at the back on the engine, and between batter and body
9) coil is strong
10) no big air leaks - i've even rigged simplified air hoses to avoid air leaks from anywhere else - note that the simplified air hoses are unlikely an issue or causing lean situation as i run the same setup on my daily driver 944 without issue.


my theorys / questions

a) my only thought was is the ecu fring the spark at the wrong time - causing a timing issue? but this seems unlikely
b) the temp sensor seems to be bad (the one with the single connector) - but would this cause a no start situation? i will replace and test anyway


thanks in advance for any ideas - i've had a 944 for 15+ years and have never been stuck like this

[Endoman]

If you are getting a pop in the exhaust it sounds like the timing is way out or the plug leads are mixed up so it's firing on an exhaust cycle.

[Wh1t3Kn1ght]

What about the speed and reference sensors down the back above the flywheel? These are usually the culprit in a no start / poor running scenario after you've checked the obvious ones.

[cabbage]

thanks for the suggestions

@endoman - i did think that, but plugs are correct and order and distributor side is also correct

@Wh1t3Kn1ght - i've tested these and they are okay, i've even swapped with working units and it made no difference sadly

[cabbage]

following @endoman comment, are there any other ways the engine timing can be so off? Does the ecu trigger the coil - and is there a chance this is out for whatever reason?

[Wh1t3Kn1ght]

thanks for the suggestions

@endoman - i did think that, but plugs are correct and order and distributor side is also correct

@Wh1t3Kn1ght - i've tested these and they are okay, i've even swapped with working units and it made no difference sadly

Are they gapped correctly?

[cabbage]

@Wh1t3Kn1ght - gaps should be correct but i will check this

[Wh1t3Kn1ght]

@Wh1t3Kn1ght - gaps should be correct but i will check this

Does the tach bounce when cranking the engine over? if so then the speed / reference sensors should be good - more info on clarks garage if you haven't already been there

http://www.clarks-garage.com/shop-manual/ign-02.htm

[cabbage]

@Wh1t3Kn1ght - yes tach bounces, and yes i've been through all the points on clarks garage already sadly

[Waylander]

to double check the timing,

take out no1 plug using a long wood kebab stick and the timing marks, find TDC on the fire stroke, get out you tipex bottle and mark the crank pulley and front cover, accuracy not relevent,

then use an old school timing light on no1 when cranking and it should light up the marks pretty close.

not definative but will give you a good idea if its right or not,

[cabbage]

@Waylander - thanks, thats a very good idea, i'll give that a try

[cabbage]

So i have done some more testing and i have one question

i tested both reference sensors resistance from their plugs, and from the ecu plug and the values match and are within range

However i've also been told that i can measure the voltage when testing the reference sensor from the ceu plug, like in thie video below:

https://www.youtube.com/watch?v=1WuZuPcVuqM

However i do not see any variation in voltage when cranking over (measuring from the ecu plug as i know this connection works)

am i correct is presuming this? and is it possible for the sensor to have the correct resistance but not actually blip in voltage?

fyi - i do get tach bounce

thanks guys

[cabbage]

correction - i do get tiny voltage reading from both speed and reference sensors on crank - but very low, is this just the meter being slow to respond?

[cabbage]

Picked up an oscilloscope to test the signals going to ecu - and they are very low, what could this be? the sensors are good and distancing from flywheel is correct.

Where does the voltage come from? is it generated by the sensor itself?

[Waylander]

They are Hall effect sensors so they generate a very low ac voltage

Very similar to abs sensors, if you see .5 of a volt I would think that’s about right