As some of you may have deduced, I've been undertaking a top end rebuild on the 355 (5.2) over the past little while. Everything has gone to plan (with some help from members on here) but now its time to drive it, something isn't right. All I can explain it as is similar to a misfire. Feels like its running on 7 cylinders. Its got brand new plugs, coils and Magnecor leads so don't think its electrical. A hot compression check shows consistent levels across all 8 cylinders (within 3-4 psi of each other). As I've presumably got spark and compression, this now has me thinking I may have a fuelling issue. So, is there a way I can check this? Does anyone know what the OBD values for the O2 sensors and also fuel trim should be? Can I do something as simple as point an IR thermometer at the factory headers to see if there's something thats not firing? Really struggling a bit. Sodding car...
I’d start with these then. If you are minded you can rig up a test for them, if not it might be worth getting them professionally looked at.
Yes you can check that voltage is being applied and a good place to start. However, even if there is voltage you may have a poorly performing injector.
Does it happen all the time? Idle, cruise and accelerating? Or only under heavier load? Heavier acceleration? Please explain when you experience the “misfire” during your test drive
Startup sounds OK ish. Its once it enters closed loop is when things seem to go wobbly with a sort of thrum... And at cruise / light throttle it feels OK. Just under load its crap
Any codes? Any pending? While it is running start jiggling connectors starting with the fuel injectors, maybe a connector is loose.
While it’s warmed up and running unplug each injector and see the result. That was how I found mine I think when first fire it up you might be able to temp gun the exhaust manifold and see which one is out a bit. Take to a dealer or a good friend with an sd and we can check each injector
Well spent another 30mins, put new fuel in, warmed up then systematically unplugged each injector. Instantly became more lumpy (almost more of the same if that makes sense) but they all seem to be working. Back to the drawing board...
Yup, grounds checked. Think its time to bite the bullet, pull the engine (again) and re-check all the timing. Having a bit of a a downer moment right now.
Just a thought, if compression and what not is good, does that speak to timing not being off? Presumably if it was off the valves would not be seated and the compression would be awful certainly on some cylinders?
If I recall correctly, exhaust opening and closing early (advanced) wouldn't have much effect on compression but would power. Depends on how much the timing is off.
Effect on power yes but not seem like a misfire I would imagine as originally stated. Does not "sound" like a cam timing issue?
Did you notice any problems when warm prior to the top end rebuild? Did you do any repainting of components, then refit earths and sensors (which require good earths) on painted components. If things change when warm, then you would suspect "closed loop" engine management components of being faulty, but temperature changes affect lots of other components in mysterious ways. Faulty fuel pressure regulator? Have you checked the vacuum lines going to your fuel pressure regulators? Just wondering if heat would affect the integrity of the vacuum lines. I have heard of lines going soft and collapsing on other cars. https://www.ricambiamerica.com/car-diagrams/ferrari/v6-v8/355-group/f355-m5-2-1996/throttle-holders-and-controls.html
If closed loop triggers the problem, we're talking emissions. Let's start with what kind of cats you have. ?
Its running decats and I replaced all the vacuum lines as I went. Think I'm going to bite the bullet and pull the engine again and double check the timing. I have a gut feel that one tooth out would give me the poor running symptoms as I think everything else is working as should be. Just trying to work up the enthusiasm.
A few thoughts: It might be a good idea to look at the fuel trim data. If you don’t have the software, I can’t imagine it costing too much pay someone to pull the data for you. Once you know where the problem is you can run down the list of potential issues. Is there any chance the MAF got bumped or fell when it was off the car? Does the car run differently if you unplug the MAF? Is there any chance you switched one of the injector connectors? Perhaps you got a defective spark plug or plug lead. Do you have the old ones to refit to rule out that as a potential issue? I just completed an engine-out and can imagine the misery of having a do-over. It’s a hell of a lot of work. Hopefully it’s something small that can be remedied easily.
If your timing is out you will most likely get an error from the crank sensor because it and the cam sensor will be out of sink. You can also remove the two plenum intakes and then start it, let it run for a few seconds. Check for exhaust,unburnt fuel coming from either bank as a light smoke. If the engine is out of timing one bank will have smoke coming out of it.
Usually not all will be exactly the same, you are sure unplugging each one exhibited exactly the same result. I had a similar idle problem and it was related to the ICV not being sealed as it had popped out. Take the ICV out and restrict its air once the car starts
