Hey guys can someone help me read my vacuum test run on my red truck? It backfires from intake. Timing is set with spout out set correctly.. Check this video on youtube i made looking at the gauge. My main question is it a normal reading or problems???? thanks!!!
btw its a 302 5 speed
YouTube - Video 0024

 

If I hear it right, kinda hard to tell exaclty on a video it pops out the intake and the needle fluctuates badly over zero vac right near the end of the video, right before you rev it higher taking off?

If so first thought is yea looks like you have a sticky intake valve.

Post a video of it sitting at an idle, will it do it at an idle? if so see if you can catch it while its backfiring.

Other then that, what I see so far looks normal. Lower reading under acceleration, higher under deceleration. Should level off and read steady at steady cruise speed level ground but no not as high as at an idle with TB butterfly fully closed.

 

Same thoughts here too. What does it do at idle? I heard some surging, I suspect that is in conjunction with the "popping"? Some questions: Stock cam/heads/intake? Have you tested each vacuum circuit with a hand held pump/gauge? Have you checked all the vacuum reservoirs for integrity? Have you pulled the codes to see what the computer thinks? What other engine mods? Overall your video shows the truck is working normally.

 

This is when i have the problem only on rpms below 1200 and wot. It will pop from intake and i will move nowhere kinda dangerous at intersections i gotta baby it just right.
The Cam is STOCK Heads are STOCK Intake is Stock.
No, sounds time consuming and scary cause i dont want to break any plastic lines-i may end up trying to test this.
Thats the sucky thing i have NO codes the cel came on once-egr was stuck tight and not moving fixed so light does work. Truck has NO mods. im going to get a idle video for you guys i have a feeling its interal its does this since iv had the truck going on a year and a half so ......

 

Normal reading, I'll give you 500 for that engine, too bad you don't have the automatic AOD transmission

 

This is when i have the problem only on rpms below 1200 and wot. It will pop from intake and i will move nowhere kinda dangerous at intersections i gotta baby it just right.

Does it "pop" out the intake every time you "floor it" without fail?

If so you more in likely have a burnt valve, only now and then, likely just a sticky valve. Reason it sticks could be a number of reasons, week/broken spring, bent valve, badly worn guide etc etc....

Still like to see video of the vacuum reading while running at an idle (motor up to running temp).

If it is a burnt valve for example yea the motor looks strong based on the vacuum readings you posted so far, so well worth a head/valve job if that in fact that is what it needs.

 

ok guys here it is i started her up for ya she was cold, but you can get the idea what is going on here. It does this no matter what, it idles beter when it warms up as you can tell by the end of the clip.


YouTube - Video 0025

 

At the moment I'm leaning toward a weak valve spring? maybe one is broken but still in place trying to keep up, cracked and somewhat shorter but still basically intact.
High vac, "idle" vacuum should be working against a week or broken spring, but at the same time the higher rpms might be leaving it falling behind the 8 ball, valve doesn't have time to close all the way.

But hard to tell exactly what the motor is doing on the video though, when you floor it does the rpm's climb and climb fairly high or does it just bog down, just flutter when you do that?

 

yeah sorry my camera phone is junky its just fumbling and boging rpms never increase unless i slowly accelerate

 

The fact it does not increase RPM's changes things a little.

I see the vacuum steadily falling when you first start it but you give it gas to save it each time? at least that is what it looks/sounds like.

Does this truck still have its Cat? and how many miles does it have on it?

 

Back fire thru the intake usually means that the engine is running lean. Try removing the vacuum hose going into the Fuel Pressure Regulator and see if there is gas on the vacuum side of the FPR-if there is---replace the FPR.

It seems to me that this truck is running lean when accelerating. Much like a carbureted engine that runs lean when the accelerator pump in the carburetor isnt working and causing a lean condition during acceleration. The FPR in these fuel injected trucks serve the same purpose as an accelerator pump in a carbureted engine .

 

I am not sure that a failed FPR would cause a lean condition on acceleration-when I think about it. An operative FPR would increase the fuel pressure during periods of low manifold vacuum-like WOT. But it seems that most failed FPR cause a rich condition--from reading other threads on this website.

Apply vacuum to the FPR and see if it holds. You should also check for trouble codes in the computer and check fuel pressure with the FPR vacuum hose connected and disconnected.

Whatever the trouble-it would seem to be caused by a lean condition when the throttle is opened suddenly.

I have heard that a constricted exhaust system can cause a steady decrease in manifold vacuum at sudden high throttle openings. How is the constant high engine speed performance of the truck??