LT5 wont run with all cylinders

[Mika73]

I guess many have already read of my problems from CF forum. But problem continues and we dont know whats wrong and what should be tested next when it feels like everything is tryed and tested. It just wont work.

http://forums.corvetteforum.com/sho...283&forum_id=50

Cylinders 2 and 7 wont work ok.

There is not ok spark on cylinders 2 and 7. There is spark, but not like it should. Spark is discontinuous.

Both O2 sensors show full reading (really rich).

I been told that it would be like winning on lotto if both O2 sensors would go at same time.

- Shop manual diagnostics done
- Dis and ECM changed
- Coil places changed
- Injectors tested (Flow ok and resistance 12.3-12.5ohms)
- Cylinders pressure tested about 200lbs
- Sparkplugs changed and plug wires tested

Thanks
Mika

[Kevin]

I was going to say injectors but you said you checked them.

[Tyler Townsley]

Anybody have some frequent flyer miles they can give Marc H to go over and get this fixed? LOL

OK I am going to go through what I would do to get a handle on where the problem is.

1. Get a new sparkplug and pull the plug wire for cyl # 1, put the plug in that wire and lay it on the plenum and start the car. Look at the strength of the spark and then do the same for all the cyl.

According to what you say cyl 2 and 7 should show weak spark, but you need to check them all.

2. Pull the plenum, first make sure the right plug wires are on the right coils, make sure the connectors are tight to the coil pack, you could pull the driver ckt plug and make sure there is no corrosion(Also check the coil conections for the plug wires as they also corrode). Then take the plug wire that showed the strongest spark and swap it for the #2 and put the #2 wire on that cyl. Pull the fuse for the injectors put new plugs in each wire ground the plugs and crank the motor, did the weak spark follow the plug wire? Do the same for cyl # 7.

3. Assuming the above did not change the weak spark location, swap the 2-3 coil for the 1-8 coil and repeat the spark test, do the same for the 4-7 coil. If the weak spark stays on the same cyl during the tests then you need to recheck the DIS module and wiring from the DIS to the coil packs.

Note the above requires no elaborate test equipment until you check the DIS and should take no more than about 2 hours.

Tyler

[Mika73]

Hi Tyler

DIS and ECM are changed: No help

Service did change two coils places: No help

They checked plug wires: No help

Sparkplugs changed: No help

Not good spark on cylinders 2 and 7. Problem stays there and O2 sensors show max reading.

And lets think time 4 weeks ago when problem started.. I had got my car back two days ago of headers, rt cats and Corsa install, driven 200km / 120miles after installation.. Suddenly on freeway at 120km/h (75mph) speed car starts shake really bad.. I had 6th gear on.. Shift to 5th did help.. It wont idle ok and wont run ok.. And service finds out that cylinders 2 and 7 dont have good spark and O2 shows max. Something happend on that freeway.. I was only cruising..

What next.. They have pulled plenium many times, but back on and checked again and again..

I forgot to add that fuel pressure is also checked: OK. My other fuel pump keeps little noise, but pressure ok.

[Mika73]

Whats common with cylinders 2 and 7 ? Do those use same coil?

There has to be something common.. Something that did break on that freeway..

[Tyler Townsley]

Originally Posted by Mika73

Whats common with cylinders 2 and 7 ? Do those use same coil?

There has to be something common.. Something that did break on that freeway..

They are not on a common coil. If the cyl are not firing correctly the O2 sensors will show rich, they are sensors and until the car goes closed loop they are not in the fueling loop.

Spark timing starts in the DIS, the spark itself is generated by the coils and carried by the wires to the plugs. The check above should isolate the problem to a specific component.

Does the car run bad from first start or does it start running badly after a period of time? I know its frustrationg for you when we cannot give you the fix but its also very hard for us as we have certain check points when diagnosing problems and without the car right in front of us we loose the ques we use to move on to the next stage. Reading what you wrote concentrates on the spark side but according to your techs everything in the spark side has checked out ok. Something does not compute so its time to recheck what has been done.

Tyler

[Tyler Townsley]

Deleted.

Tyler

[Mika73]

Latest information:

Engine runs ok with headers only. Service did remove catalysts and it runs fine.

My RT cats were used only 120miles.. and two days.

What happend..

[Kevin]

that is weird. I don't see what the cats have to do with a miss

[95ZR1]

What did you do, put leaded gas in that car? I wonder why they kept saying 2 and 7 weren't firing right if the cats were bad.