Engine fails to start after rebuild.

[mgbloke]

Hello Guys, bit of a weird one. 18 months ago I removed my 5.0L RV8 to change the big end bearings and fit a dry sump system. Now its all back together and am having issues with popping back through Throttle bodies 5 & 6 when cranking and being really hard to start. I did eventually just get it running and it seemed ok pickup was good and seemed smooth. Got it up to temp and switched off. next day I tried to start and same thing only wouldn't start at all. Popping back through 5 & 6 when cranking. When I removed the air filters 5 & 6 inlet tracts were black. Running a Link G4+ extreme ECU which was mapped on a rolling road and has a known good setup. I checked the Trigger offset to see if anything changed when i fitted the pulley for dry sump pump and all OK.
When the heads were off ( TA big valve) they looked fine. Planning to do a compression test to see what pressures are. I'm wondering if I have somehow cocked up the cam timing?
Any thoughts would be appreciated.

cheers

Mark

MGB GT V8 5.0L

[stevieturbo]

Exactly what was changed during the "rebuild" ? Only bearings ?

Presumably you have a crank trigger on there....can it only be installed in the correct place ? Have you verified timing you are expecting, is timing seen at the plug ? Do you still have a dizzy ? What ignition setup ?

Did you do anything strange with the lifters, pushrods or rockers during the rebuild that might cause valves to stay open ?

What was the reason for the rebuild in the first place ?

And when things are apart, yes it is possible to make a mess of a lot of things including cam timing. Although that would then impact all cylinders, not just 2.

[DaveEFI]

I'd use a timing light to verify the spark is occurring when it should be. What sort of ignition system do you have?

[ChrisJC]

Stuck injectors?


Chris.

[mgbloke]

Thanks guys,
OK it is sequential fuelling and wasted spark ignition. 32/1 trigger wheel on crank single tooth on cam.
I took engine out to fit new dry sump system. Big ends replaced as were fairly worn.
The trigger wheel was removed from crank pulley and re fitted with addition of oil pump pulley. Checked trigger offset with a timing light and is same as before. I checked valve clearances which were slightly out but not enough to hold a valve open. The link uses an external ignition amplifier so will change to eliminate.
Lifters and pushrods exactly the same as before.
Not sure what to do next.

Cheers

[Ian Anderson]

Few things spring to mind.
Battery low on power after 18 months. Perhaps add jump leads to crank till it runs long enough to fully recharge.
Spark leads, is the right one going to the right plug?

Ian

[stevieturbo]

Thanks guys,
OK it is sequential fuelling and wasted spark ignition. 32/1 trigger wheel on crank single tooth on cam.
I took engine out to fit new dry sump system. Big ends replaced as were fairly worn.
The trigger wheel was removed from crank pulley and re fitted with addition of oil pump pulley. Checked trigger offset with a timing light and is same as before. I checked valve clearances which were slightly out but not enough to hold a valve open. The link uses an external ignition amplifier so will change to eliminate.
Lifters and pushrods exactly the same as before.
Not sure what to do next.

Cheers

Pointless changing parts with no reason. Check the spark on all cylinders.

Is the ecu reporting any crank/cam trigger errors ?

Do a compression test. What do the plugs look like ? What are mixtures like on each bank ?

[minorv8]

I had similar issues with my new engine. I run an older Haltech ECU with wasted spark. For no apparent reason the engine started poorly, ran only on 6 cyllinders. Previously no issues. Finally it "ran" only on 4 cylinders if you can call it running at all. I had set the valve clearances and suspected a mishap there. I did a compression test and all cylinders were OK. So I knew it was not an mechanical issue. I checked the exhaust manifold with infrared gun and found out that all 4 dead cylinders were connected to the same wasted spark coil (Bosch 407 like in P38´s). Another coil did not fix the problem, a new ignition module did. Don´t know what killed it, they are supposed to be reliable. Both modules have a large heat sink so heat is not an issue.

[GDCobra]

Is it possible plug leads for 5 & 6 have been transposed?
These cylinders don't relate to the same coil and presumably their opposite numbers (1 & 8 ) are/were firing OK which would indicate the coil and ignition module are working.

[DaveEFI]

If you had the engine out to do the work I'd be double checking all the connections to the engine loom. Injector connectors swapped by mistake? Any damaged? And so on.

[mgbloke]

Recently replaced the battery ( last week) so its not that. It is impossible to get the injector connectors wrong. plugs 5&6 are fed from separate coils.
I did move the location of the ignition amp slightly so will double check the wiring but connectors only fit one way. Will pull some of the loom apart to check. Borrowed a compression tester so hopefully get that done tomorrow. Plug wires are correct.
No errors reported by ECU. I will also run an injector test to hear if any are stuck open.

Thanks for the suggestions.

[GDCobra]

. plugs 5&6 are fed from separate coils.

Yes, I undesstand that hence my comment, if they were a pair from the same coil then it could be the coil or the driver in the ignition module but as they are not related then (if it is an ignition problem) the issue must be between the coil and the plug (if each cyliders matched pair is working OK) which only really leaves the lead ordering or the plugs & leads themselves. As 2 cylinders seem to be affected the most obvious would be that their leads are transposed.

[stevieturbo]

Recently replaced the battery ( last week) so its not that. It is impossible to get the injector connectors wrong. plugs 5&6 are fed from separate coils.
I did move the location of the ignition amp slightly so will double check the wiring but connectors only fit one way. Will pull some of the loom apart to check. Borrowed a compression tester so hopefully get that done tomorrow. Plug wires are correct.
No errors reported by ECU. I will also run an injector test to hear if any are stuck open.

Thanks for the suggestions.

If an injector was stuck open, there would be cylinders absolutely flooded, maybe a hydraulic lock, and plug etc soaking. I presume none of that has happened ?

[ChrisJC]

If an injector was stuck open, there would be cylinders absolutely flooded, maybe a hydraulic lock, and plug etc soaking. I presume none of that has happened ?

I had a stuck open injector. When I first fired up the engine, it was OK, but clearly misfiring on one cylinder on account of it being very over-rich!

After I switched it off, but then allowed it to sit there with the ignition on but not running, it did indeed fill the cylinder with petrol and suffered hydraulic lock. The petrol does run into the sump, so the oil level goes up, and smells very much of petrol!!

Chris.

[mgbloke]

Update. I did a compression test that showed all cylinders low 90-100psi.
Checked the cam timing which was 12 degrees out so inlets were slightly open when firing. Hoping valves are OK.

Cheers mark