My F150 suddenly decided to stop idling and I suspected a vacuum leak or a bad IAC sensor. No DTC codes to help. Checked all vacuum lines coming out of the upper air plenum that sits on top of intake manifold. After more closer inspection, I found the source:


Note the dangling piece of gasket hanging below the blue spark plug wires and the plastic vacuum hose going to brake booster. Back pressure from engine forced the plenum gasket to rupture.

 

Interesting. First thoughts are whether the gasket was installed properly (has to be aligned a certain way) and whether the intake was torqued down properly-one round at 89 in-lbs and the second at 53 in-lbs if I recall correctly.

 

Only turbochargers and intake backfires cause intake manifold pressure. Have there been any backfires? 2000 F150 4.2L, right?

p.s. I don't see how the gasket could get out of its groove unless it was improperly installed, or the manifold broke. Will be interesting to see what you find. Have fun getting those back bolts out. A 1/4" drive ratchet and a wobbble socket might help.

 

Yes, 4.2l engine, 2001 F150 with 178k miles. I've had occasional backfires in the past. I think it was installed correctly but the backfires can screw up the sealing areas. Last year one loud backfire was so strong it ruptured the upper air intake manifold assembly. See photo:

I kept this as a show and tell souvenir. I was very meticulous about torque sequence and specs after this loud explosion/backfire which cracked the plenum last year. Note gorilla glue and the hairline crack going to throttle body. 25 inch pounds are the torque on outer bolts, I believe. Replacing the housing was fairly expensive, over $200 from rock auto if I recall correctly. There are six vacuum connections here FYI.

The backfiring is likely from the bad IMRC bushings which I have so far neglected to replace. Engines runs lean because the IMRC butterfly valves are stuck open. I recently found one of the control rods hanging out in back by firewall. Hence the backfiring condition, which I know now is an extremely self-destructive event that you want to avoid at all costs. The force of the backfires must be slightly lifting up the upper housing enough to rupture the seal and push it out of its seating area.

 

The missing bushings should cause an air leak but the PCM should compensate with more injection time. And the stuck IMRC butterflies could affect performance but shouldn't cause backfires. I wonder if you have some other issue causing the backfires. Have you been driving with the SES light on?

Thanks for the picture. What's your plan? Good luck with it.

 

Yes, I've tried addressingall the DTCs addressed by OBD2 scans to get rid of check engine light. New plugs and wires. Swapped out coil pack half a dozen times. I've used the shotgun approach and replaced upstream/downstream O2 sensors, EGR, EVAP, pretty much most sensors have been replaced, including cam and crank position sensors. Light just won't go away. I haven't done anything with fuel injectors and fuel pressure regulator yet. Fuel rail pressure is normal, fuel pump in tank has been replace three times in the last year, and fuel filter is recent. Fuel pressure is acceptable as far as I can tell. Would running on higher octane help to minimize backfires?

I probably have some old evap hoses connecting to gas tank that should be replaced. I get so used to dropping the gas tank when replacing fuel pump, that I don't really tighten down the hoses going back into gas tank. The hoses more or less slip on and off for easy R&R of tank. Wouldn't think that would contribute to backfires. It probably timing related. Timing chain is original but engine normally starts up and run very good at highway speeds that I see no point in yanking out a chain for no good reason,

 

What is/are the code(s)? That's the key.

How many miles on the engine?

 

Not that I recall…and I have done this repair three times on my old 4.2 before I got a younger one swapped in last spring (changed all this stuff on the pallet before that install as well). That figure doesn’t ring a bell.

It’s a uniform torque spec for each pass on all bolts. That would explain what you’re showing. The link below kinda explains that, but the Haynes manual will definitely show what you need to know as it looks like online illustrations are disappearing now.

As for IMRC butterflies, they’re generally harmless. Even with the old engine having blow-by and forcing bank 1 open non-stop, all that did was cause boggy acceleration. I guess weirder things could happen, but that’s not their track record.

https://www.ford-trucks.com/forums/8...ake-bolts.html

 

I think there are IMRC lockout kits for the Mustang Coyote 5.0l engines. So yes, I agree that replacing IMRC bushings need not be an immediate priority. Since I'll be removing the upper intake assembly, it would be the best time to tackle this job while installing new gaskets.

BTW I made a special tool to remove/install those those two difficult to access 10mm bolts hidden between intake housing and the firewall. Basically you hammer on a deep well 10mm socket into the closed end of a 15mm crescent wrench. You only have 4 inches of clearance to work in that restricted area. I'll upload the photo on my next post as I'm outside with truck in the dark with drizzling rain at the moment.

 

I took another photo of the bad gasket for the 2001 Ford F150 'throttle body spacer' (Motorcraft PN 5308-05240647 or possibly Motorcraft 1L3Z9A589DB). Note: the matching upper intake plenum seems to be much easier to find than the lower half.

If the gasket had ruptured internally and remained tucked away inside the spacer (aka lower intake manifold plenum), the vacuum leak would not be visible and could have only been detected by spraying starter fluid in that area, then observing an increase in RPMs in that specific area.

 

I took another photo of the bad gasket for the 2001 Ford F150 'throttle body spacer' (Motorcraft PN 5308-05240647 or possibly Motorcraft 1L3Z9A589DB). Note: the matching upper intake plenum seems to be much easier to find than the lower half.

If the gasket had ruptured internally and remained tucked away inside the spacer (aka lower intake manifold plenum), the vacuum leak would not be visible and could have only been detected by spraying starter fluid in that area, then observing an increase in RPMs in that specific area.

 

Some places call it a plenum gasket. They call the middle piece a "spacer". And the metal bottom the manifold. Makes parts searching difficult.

Kind of looks like installation error. That top or cover is hard to slide in there.

https://www.oreillyauto.com/detail/c...t/fel0/ms94452

 

I could have pinched the gasket from the previous install, can’t rule that out. The occasional backfire doesn’t help.

I had an extra gasket set to finish the job today. I’m actually getting 19 in/Hg of vacuum at idle with warm engine, that’s a slight increase over the 17-18 in/Hg I was getting before the ruptured gasket.

i also took the opportunity to replace the 4 white IMRC bushings that connect both ends of control rods. I had to remove fuel rail to get into those tight spots. The Dorman bushings were a little brittle. and I don’t expect them to last long. I can’t tell a big difference in performance but thought I’d at least try to put them in while manifold was removed.

 

Any chance you could post the codes that were caused by the problem?

 

Yes, I pulled up some dtc codes on one OBD2 scanner: P0340 (cam postition sensor), P0443 & P1451 (evap emissions). No lean codes on this scanner. OTOH I used a different scanner that gives me different dtcs, including o2 upsteam and downstream faults ( replace all O2 sensors last month). So which do you believe? At least I don't seem to be getting 'too lean' codes, i.e. P0174, P1518 (IMRC stuck open), and P0171 (system too lean). These codes i pulled with a scanner from autozone a few months ago. Camshaft postion sensor has been replaced recently, so something must be triggering this code that is unrelated to that sensor itslef. OBD2 scanners are more accurate for newer cars with fresh wiring harnesses. My 2001 F150 w/178k miles has been around the block too long to rely solely on scanner DTCs.

I think my gas mileage is improving with the new IMRC bushings and the new manifold gaskets My vacuum guage is at this time consistently pulling higher vacuums than previous, at all speed ranges. BTW Those "fuel economy guages" on the market are just glorified vacuum guages. You want to be running with engne vacuum at highest range under all engine loads for best economy. Haven't got any backfires yet. A lean engine can create a backfire if PCM doesn't compensate for the extra air in combustion chamber.