Pressure in intake question

[bdinardo]

Hi all,
I recently rebuilt the top end of my 1979 Fuel Injected 2000. Upon starting it for the first time after the rebuild, it did in fact fire up for a second or two but then the intake plenum (big plastic piece between the air filter box and the metal intake manifold) popped off the large intake manifold connection overcoming the hose clamp. Any ideas what would cause that much pressure build up? During re-assembly, I was careful to ensure that the motor was at TCD (firing cycle for cylinder 4), the cam gears were both lined up with the pointers on the cam timing marks and the distributor was pointed to fire on cylinder 4 +/- a few degrees. I'm cautious not to try starting it again until I have more of an idea on what's going on to ensure I don't harm the engine. Any advise would be greatly appreciated!

Brian

[18Fiatsandcounting]

That is odd. Did you hear any backfiring sound when it did this? Here's what I would do:

1. Recheck the timing of all the gears on the timing belt circuit (TDC, camshaft, aux. shaft).
2. Pull the spark plugs and do a compression check on all 4 cylinders.
3. If the timing and compression are both good, try to start it again.

If you installed the head with the #1 and #4 pistons at TDC, it's possible that you bent an intake valve as you lowered the head onto the block. It doesn't take much to bend a valve with sideways deflection, and it's why I put the pistons in the middle of their stroke when reinstalling the head, and then rotate the crankshaft into position when the cams are lined up. Prevents inadvertently knocking (and bending) a valve on a piston crown. But, if your compression is good on all 4 cylinders, this isn't the problem.

-Bryan

[bdinardo]

That is odd. Did you hear any backfiring sound when it did this? Here's what I would do:

1. Recheck the timing of all the gears on the timing belt circuit (TDC, camshaft, aux. shaft).
2. Pull the spark plugs and do a compression check on all 4 cylinders.
3. If the timing and compression are both good, try to start it again.

If you installed the head with the #1 and #4 pistons at TDC, it's possible that you bent an intake valve as you lowered the head onto the block. It doesn't take much to bend a valve with sideways deflection, and it's why I put the pistons in the middle of their stroke when reinstalling the head, and then rotate the crankshaft into position when the cams are lined up. Prevents inadvertently knocking (and bending) a valve on a piston crown. But, if your compression is good on all 4 cylinders, this isn't the problem.

-Bryan

Well I think you're on to something as I did a compression check and here were my results. 1: 139psi, 2: 140psi, 3: 121psi, 4: 90 psi. Clearly something is off with cylinder 4 so let disassembly begin....again.

[18Fiatsandcounting]

...I did a compression check and here were my results. 1: 139psi, 2: 140psi, 3: 121psi, 4: 90 psi.

140 psi is about what I would expect for a stock engine, so #1 and #2 would appear to be OK. 121 psi isn't terrible, but nor is it good. 90 psi is definitely low, but it the valve were bent (and thus not closing), I would expect close to zero.

Another possibility is that the head gasket isn't sealing between bores #3 and #4, so that combustion in one of them could lead to a pressure pulse in the other, and if that other cylinder happens to have its intake valve open slightly, well, that could also explain things.

So, check for a bad valve but also carefully inspect the head gasket and flatness of the block and head, especially around cylinders #3 and #4. Did you have the head lightly "skimmed" before putting it back together? Always a good idea, although this does require a machine shop.

-Bryan

[Nut124]

Did it have good, even compression before the head rebuild?

Incorrect ignition timing can cause the incident you described. Did you try to check timing with a light? I would turn the dizzy CW about 10deg and try again.

[bdinardo]

Did it have good, even compression before the head rebuild?

Incorrect ignition timing can cause the incident you described. Did you try to check timing with a light? I would turn the dizzy CW about 10deg and try again.

Before rebuild, all cylinders were at 140 +5psi.

I did not get the car started long enough to check ignition timing as the pressure build up in the intake cause enough concern not to try and start it. I'm going to take the head off and check valve train. I'll report back findings.

[18Fiatsandcounting]

I'm going to take the head off and check valve train. I'll report back findings.

This may be too late, but before you pull the head again, I would adjust the ignition timing as Nut suggests. As he noted, if the timing is way far off, you can get a small explosion when an intake valve is still open, perhaps causing what you observed.

Hate to have you pull the head if that isn't the problem. There is still the problem with the uneven compression numbers, but I'd get it running first before addressing that.

-Bryan

[Nut124]

The loss of compression is worrysome.

It takes quite a bit to bend valves and you would have to have bent 2. Do you have any recollection of an incident that could have caused this?

If you remove the head, I would look for any evidence of fire crossing the narrow land between 3 and 4.

Potential causes for this would be head bolts being stretched or too long. What head bolts did you use?

When re-installing the head, first, as a test, insert all head bolts w/o the washer and make sure you can drive them in far enough to touch the alu head w/o excesive torque. Do not tighten on the head. This way you know the bolts were not bottoming out in the threads. Re-install with the washer.

[bdinardo]

The loss of compression is worrysome.

It takes quite a bit to bend valves and you would have to have bent 2. Do you have any recollection of an incident that could have caused this?

If you remove the head, I would look for any evidence of fire crossing the narrow land between 3 and 4.

Potential causes for this would be head bolts being stretched or too long. What head bolts did you use?

When re-installing the head, first, as a test, insert all head bolts w/o the washer and make sure you can drive them in far enough to touch the alu head w/o excesive torque. Do not tighten on the head. This way you know the bolts were not bottoming out in the threads. Re-install with the washer.

Quick response and update. I purchased new head bolts and full gasket kit including the head gasket from Autoricambi as part of this rebuild and they were the same length as old bolts. The timing belt was off by 1 tooth upon initial inspection.

I ended up taking the head off again to confirm a few things. In short; no sticking valves as they all moved up and down as they should with correct shim clearances. Head gasket appeared good as well with no evidence of cross combustion between 3 and 4. I installed and torqued down the intake and exhaust manifolds to the head to ensure they were all torqued correctly (much easier to do on the bench vs. in the car). I put it all back together and tried again this time with the timing belt in correct location, and compression values improved. 140 for cylinders 1 and 2, 130 for cylinder 3 and 120psi for cylinder 4. While I'm not thrilled with the large range it should be sufficient to start the car.

Same outcome unfortunately, where the car wont start and I had the pressure build up enough to blow off the plenum boot again. Lots of turning over with little to no firing. Any other ideas?

Question: Where is a good starting point for the distributor. If I put the cam and crank back at TDC again where should the rotor be pointing roughly?

[bdinardo]

Tinkering a bit more tonight revealed a leaky fuel hose that went from the tank to the in line electronic fuel pump. I replaced the hose but no change. I also confirmed spark as I put a timing strobe light to cylinder 4 and while cranking got the strobe to blink. Car still will not fire up, just turns over. Upon turning over for 30 seconds or so, will build up enough back pressure in the intake to blow off the intake plenum again. I'm thinking now I don't have enough fuel pressure building up since I've confirmed spark and compression so what's a good way to test fuel rail pressure or if my fuel pump is working?

[Nut124]

My goodness! You seem to have something backwards, don't you?

Are you saying that the intake boot does not blow because of a single misfire but because pressure building up during sustained cranking?

If so, then the cams must be totally out of time somehow.

As to how to set the dizzy; Locate the #4 electrode on the underside of the cap. Trace it down to the rim of the cap and mark with a marker. Transfer the mark to the dizzy aluminum base. With crank at TDC and cams at the index mark, set the dizzy so that the leading edge of the rotor is lined up with the mark. The rotor turns CW.

Trace all plug wire s from plug to dizzy # to make sure some PO did not set it to time on #1.

I could set mine at TDC and remove the cam covers and post pics of cam lobe positions for you to compare to.

[Nut124]

Cam alignment:

Looking from the front, the cam lobes at TDC, #4 firing, should be pointing to approx (clock hour arm positions):
I1: 5:30
I2: 8
I3: 2:30
I4: 11
E1: 7
E2: 9:30
E3: 4:30
E4: 1:30

Pictures at TDC, #4 firing:

[bdinardo]

My goodness! You seem to have something backwards, don't you?

Are you saying that the intake boot does not blow because of a single misfire but because pressure building up during sustained cranking?

If so, then the cams must be totally out of time somehow.

As to how to set the dizzy; Locate the #4 electrode on the underside of the cap. Trace it down to the rim of the cap and mark with a marker. Transfer the mark to the dizzy aluminum base. With crank at TDC and cams at the index mark, set the dizzy so that the leading edge of the rotor is lined up with the mark. The rotor turns CW.

Trace all plug wire s from plug to dizzy # to make sure some PO did not set it to time on #1.

I could set mine at TDC and remove the cam covers and post pics of cam lobe positions for you to compare to.

Nut124, thanks for the information and pictures. My timing is set up exactly as you've described and pictures show. Crank at TDC with pointer directly over the notch in crank pully and pistons of cylinders 1 and 4 all the way up, holes in cam gears lined up with the pointers to put the cam shafts in your matching positions (cylinder 4 valves both closed for intake and exhaust), and distributor rotor pointed at just before #4 electrode.

Additional new information. I've removed the black tube between air filter/air flow box and intake manifold and began cranking the engine. After about 20 seconds I saw a singular backfire come out the intake manifold/throttle body confirming that it likely isn't a slow build up of pressure but rather one backfire. I've started to perform the checks outlined in the Bosch J-Injection Diagnostic Manual but so far nothing is rearing it's head and I still can't get this car to start!

Battery is new and fully charged, new fuel, fuel pump comes on when I deflect the AFM arm, timing strobe light on plug wires during cranking produced correct light flickers. Next on the list to check is fuel pressure at the rail, injectors firing, and various potential electrical gremlins. Any other thoughts are appreciated.

[Nut124]

Firing into intake to me points to ignition timing being way off. I would check that carefully.

[bdinardo]

All,
I figured I'd give an update to this thread. 2 issues were found when I ultimately took it to the mechanic. One was indeed the ignition timing which was off by about 15 degrees. Secondly the brand new fuel pressure regulator was bad and only allowing about 20 psi of fuel pressure when the spec is 35 psi minimum I believe. So we swapped out the fuel pressure regulator for a used Alfa Romero one he had laying around and with those two things combined the engine runs quite well. 900 RPM and temperature holds steady at 190 degrees. Thanks for everyone's input. I'm just glad the head work I did was sufficient to run this car. Took the family out on the car's maiden voyage around the neighborhood last night since the car has been sitting 5+ years ago!