97 Suburban Vortec 5.7L vacuum leak?

[shovenose]

My Suburban ran GREAT since i bought it and replaced the fuel injector spider. But now when accelerating off idle it stumbles and seems to almost stall (though it never has). I think it's some sort of vacuum leak. I tried unplugging the MAF sensor one evening after work and drove home and it ran a lot better (engine light on, but the hesitation was much less noticeable) though the next morning it ran really bad at which point plugging in the MAF sensor didn't make much of a difference, or maybe improved it slightly.

I sprayed water and then carb cleaner all over the intake tract, vacuum lines, and intake manifold and didn't find any vacuum leaks. What should my next step be?

 

[Mooseman]

Any codes or CEL? Check the fuel trims via Torque.

 

[I_Shoot_Back]

Clean the MAF sensor often helps a lot

 

[shovenose]

Any codes or CEL? Check the fuel trims via Torque.

Fuel trims are very positive, like 20%+

Clean the MAF sensor often helps a lot

Yup, already did this, twice. Once, a few months ago before this problem happened, and once again trying to resolve this problem.

 

[littleblazer]

Did you get under the spider plug? That can leak.

 

[shovenose]

Did you get under the spider plug? That can leak.

Yup, sprayed there. When I replaced the spider I lubed the gasket/o-ring and it went on relatively easy - I've heard people having trouble with it when they didn't lube it.

 

[littleblazer]

The spider is the new style with injectors not poppets right?

It could be possible one of the intake gaskets failed and its leaking on the crank case side. You could plug the pcv and see if it improves...

 

[shovenose]

The spider is the new style with injectors not poppets right?

It could be possible one of the intake gaskets failed and its leaking on the crank case side. You could plug the pcv and see if it improves...

Yeah, it's the updated type.

Could you please explain, how would I plug the PCV?

 

[littleblazer]

Two pieces of tape over the valve cover vents. If it pulls vacuum it'll start running better. Problem is that if there is enough lockdown from the heads and pistons it'll still do it...

 

[shovenose]

Two pieces of tape over the valve cover vents. If it pulls vacuum it'll start running better. Problem is that if there is enough lockdown from the heads and pistons it'll still do it...

Assuming you mean leakdown? thanks

 

[littleblazer]

Assuming you mean leakdown? thanks

Yea. Autocorrect is wonderful.

 

[shovenose]

Alright so I have yet to try that, really need to. Won't the engine suck the tape in?

Also, this O2 sensor output doesn't look normal (this was recording during sitting in traffic and driving, I should have graphed engine RPM, but shouldn't both O2s be reading about the same?

 

[Sparky]

upstream O2 sensors should be all bouncy up and down all the time cycling. It looks like your bank 1 sensor may be getting lazy though. Bank 2 looks more normal.

 

[Mooseman]

Ditto. You could just replace it or, depending on how hard they are to get at, swap them side to side and see if the readings follow. And there's your fuel pressure issue too that could be playing into this if it's leaking fuel into the intake via the vacuum line.

 

[shovenose]

Ditto. You could just replace it or, depending on how hard they are to get at, swap them side to side and see if the readings follow. And there's your fuel pressure issue too that could be playing into this if it's leaking fuel into the intake via the vacuum line.

If I'm gonna bother taking them out they are getting replaced. I just ordered two new Densos from Amazon because at work all we sell is Bosch and they are garbage.

 

[shovenose]

Alright the new O2s are in. No change. Also changed the fuel filter today and no change there either. But back to the O2s, the ones I took out were Bosch (aka Trash in my opinion) and had white discoloration on the snout (or whatever you call it) but I'm just happy not to have Bosch O2 sensors in anything but a European car because those just don't mix...

Thing is with the MAF unplugged the truck runs totally fine. Runs pretty smooth, sounds good, no off-idle hesitation, squeal the tires in reverse easily, plenty of power (I don't have an identical truck for comparison but seems similar to every other Vortec 5.7-powered K1500 truck/Suburban I've owned and I've owned many). And fuel trims are fine, LTFT is about +8-10% and STFT varies within +/-10 but mostly +/- 0-3%, as if NOTHING is wrong with it minus the "Service Engine Soon" and MAF sensor code. Then I plug in the sensor, and immediately fuel trims are +20% and you can feel the engine isn't happy.

At this point it MUST be either a large vacuum leak (which obviously won't affect it if there is no MAF reading) or a bad MAF but the MAF is new. Oh well, I will put a third one in tomorrow... But assuming I don't have two bad MAF sensors it's GOTTA be a vacuum leak. I think I HAVE to smoke test it...

And my fuel pressure issue (separate thread) I guess has NOTHING to do with any of this.

 

[Tdub]

Just jumping in for the first time here. On the vortecs at least, if fuel pressure is under par, disconnecting the MAF sensor will bring down fuel trims some due to it operating with speed density fuel control. I've been burned by that whole "running better with MAF unplugged" before. Volumetric (MAF) control, and speed density (MAP) affect fuel trims in an opposite way as engines age. So it is possible that the system by design is compensating for lower than spec fuel because it is based off of a slightly lower vacuum or higher map, rather than the actual amount of air coming in.

The important thing to make note of is the fuel trims during a heavy load which is best achieved with a wot immediately after a 1-2 shift. Speed density control won't cover up a fuel delivery issue during that kind of load and if fuel is the problem, it should show its ugly face as increasing positive fuel trims during a heavy load.

Reading through your other post I want to mention that if the regulator is leaking, your fuel trims will dive into the negatives at idle and get better with wot. The opposite is true with vacuum leaks, high at idle, and normal with acceleration.

Also it is normal that fuel pressure should go up a little when vacuum drops (or accelerating ), but just by a few psi.

 

[shovenose]

Just jumping in for the first time here. On the vortecs at least, if fuel pressure is under par, disconnecting the MAF sensor will bring down fuel trims some due to it operating with speed density fuel control. I've been burned by that whole "running better with MAF unplugged" before. Volumetric (MAF) control, and speed density (MAP) affect fuel trims in an opposite way as engines age. So it is possible that the system by design is compensating for lower than spec fuel because it is based off of a slightly lower vacuum or higher map, rather than the actual amount of air coming in.

The important thing to make note of is the fuel trims during a heavy load which is best achieved with a wot immediately after a 1-2 shift. Speed density control won't cover up a fuel delivery issue during that kind of load and if fuel is the problem, it should show its ugly face as increasing positive fuel trims during a heavy load.

Reading through your other post I want to mention that if the regulator is leaking, your fuel trims will dive into the negatives at idle and get better with wot. The opposite is true with vacuum leaks, high at idle, and normal with acceleration.

Also it is normal that fuel pressure should go up a little when vacuum drops (or accelerating ), but just by a few psi.

Ok, I will investigate this: Reading through your other post I want to mention that if the regulator is leaking, your fuel trims will dive into the negatives at idle and get better with wot. The opposite is true with vacuum leaks, high at idle, and normal with acceleration.

Thing is, regardless of a vacuum leak, at this point either we have a leaking fuel pressure regulator or a failing fuel pump. Assuming I cut myself a fuel pump access hole I am wondering if I should just put in a new fuel pump for $35 and see if that helps. That's less work than changing the fuel spider again...