Hi Guys,

I'm at the end of my wits with my 400SBC 24x conversion.

I'm almost ready to pull the entire motor and either rebuild again or just junk the project.

I had the car running, idling sort of ok after warm up and could drive it around, but had a weird surging, backfiring spluttering whenever the auto changed, or whenever I put my foot down.

The car hadn't been tuned and was running very rich so I didn't drive it very much.

Anyway the problem has gotten worse to the point where to start the car it takes about 30 seconds of cranking and some luck with the throttle to get it to fire up.

On top of that I have about 10" vacuum when it should be 20".
The specs are:
400SBC, stock bottom end, flat pistons with valve reliefs, AFR ported heads to 220cc, HSR intake, 30lb ls1 injectors, XR270HR cam and 4L60E

All the 0411PCM wiring and pinouts are correct, harness has been repinned for SBC firing order for injectors and coils, and I've triple checked all the wiring.

There's no DTCs other than the MAF circuit as the tune is Speed Density only.

I was down to 2 possible options, a vacuum leak in the lifter galley/ intake gaskets (checked for all other vacuum leaks) or the valve timing was set wrong.

I put a rubber glove over the throttle body and taped it up. Plugged the fuel regulator vacuum line and blew into the pcv valve hose to build a bit of pressure. The glove inflated, but there was no apparent leak. The glove lost a tiny amount of air over 30,40 seconds, but still stayed mostly inflated.

I'm assuming that means no intake gasket leak?
Unless the outer area of the intake is sealed, but leaking into the engine??

There was a bit of oil on the 7 & 8 cylinder plugs when I pulled, cleaned and tested them.

Any ideas?

 

What do the plugs look like? Perhaps fouled from rich run conditions?

Also what is your actual verified timing on the balancer with a light? Does it match commanded? Maybe it needs more timing to drive better off idle and to get vacuum up.

Lastly make sure the reluctor wheel does not move around and is firmly attached to crank. Any movement will throw off timing and make engine run poorly. My setup the ring losened up the keyway slot and the wheel shifted back and forth. Was getting inconsistent starting cuz the timing was off

Lastly how good is the tune so far? Fuel right? When you experience sputtering what does air fuel do? Have a wideband?

No electrical issues with spark plug wiring? Using resistor style spark plugs?

 

Yeah it was running pretty rich, o2 sensors were sooted right up, so I soaked them in fuel and cleaned them up, but the tune isn't using them yet anyway. Plugs were pretty black and wet, I know it's running rich but we need to get it running before we can drop fuel without leaning it too much.
No sure on actual timing, I can't get it to stay running enough to check.


I haven't check that yet, but I'll take a look through the sensor hole and see if there's any wiggle. I was reluctant to take the timing cover off unless I had to as I sealed the plastic vortec one pretty well and I'll have to replace it possibly if I remove it (In Australia, so vortec parts are not readily available)


Tune is pretty average. The fuel was dialled in by a dyno shop, but that was with a massive vacuum leak from a port on the throttle body that I thought was a heater coil hose, so it would be running rich again now.


All spark plug wiring was correct, tested resistance and everything was in range.
...Resistor style spark plugs ....4 of them definitely are, the 0416-4s accel shorties, the other 4 are msd I think and not too sure if they're resistor type.
Would that cause this type of issue? I thought the resistance was more for protection of the PCM?

 

Put in fresh plugs, lean it out in the ve tables and try again. they should not be wet or blackened. Dont need o2 sensors at this time anyway

 

yeah I pulled all the plugs cleaned them and swapped out a few, similar result, but I hadn't leaned the ve table yet.
Should I also adjust cranking fuel?

 

If it cranks and starts fairly quickly i wouldnt touch cranking tables yet

 

it takes about 6 attempts and 30 seconds to a minute of cranking to get it to fire up.

 

Then yeah that area needs work. But hard to tell if needs fuel or has too much, unless after 5-8 seconds you stop and check plugs to see if wet. More Timing may help too depending where its at now

 

Ok cool.


I've got the top of the intake off, and planning to remove the intake to check the lifters, and intake gaskets this afternoon.


I may go one level further and take the heads off to check the head gaskets, valve seals and rings as well so I don't waste more time trying to fix mechanical problems with tuning.


It's my first build, so I guess there was bound to be some problems.

 

No need to remove the heads to check for a vacuum leak. Do a leakdown test but you will need a compressor for air supply.

Fix your vacuum leak first then move on to the ignition and tune. But you do have alot to learn and verify here. Did you upgrade the tune for the new injectors constants? Fuel pressure?

 

Just ordered a leak down tester and piston stop tool so I'll get the leakdown test done during the week.

Injector values are all correct in the tune.
I installed a fuel pressure gauge so I know that is set correctly and not dropping out.

 

Ok, so update:

Leak down test was showing 100% leakage at TDC on cylinder one, and found it leaking out the intake. Ended up being one of the valves was partially open, so i lossened off the rocker and readjusted. Horaaay!!......not quite

Fixed that, redid the test and still had 75% leakage. I rechecked the gauge was calibrated, and the leak down tool was set to 0 at around 12-15psi

Air wasn't coming out the intake, exhaust, radiator or dipstick but sounded like it was coming out inside the motor somewhere.

I haven't been able to find info, but should the leakdown test be done with the engine hot to allow rigs to expand etc?

If it's not leaking from intake or exhaust and is going down into the motor, but compression is still 200psi.... does that mean its ok?

The leak down tester is a cheap $20 chinese ebay deal so could it just be crap? I can hear the slight leak but not enough to pick up where from, and even when i up the pressure it sounds like its maybe leaking from everywhere, i.e. the sump and oil galleys.

Anyone?



Compression tests: cyl# 1,2,4,5,6,7,8 = 200psi cyl#3 =220psi

 

I don't know where you could use a L/D tester set to zero? Or 12-15psi? My Summit $80 tester has table for leakdown percentage at a given supply pressure: https://www.summitracing.com/parts/s...a/instructions.
Try a tester from a reputable mfr. The orifice needs to be sized correctly for the volume of the cylinder you have - not just a hole drilled in a plug.

Try again. But it reads like you over adjusted the valves/lifter preload. You should do a search for some good instructions on adjusting the lifters. Summit racing has videos on this in their tech section.

Good luck.