Loud Pop under boost. HELP!?!?!

[MY T FAST]

Hi,
first, the car is very close to stock. i have a maf pipe with a k/n, rjc boost controller set at ~17, TT 93 chip, scanmaster, and boost gauge. everything else is stock. i've had the mods on for about 900 miles and this is the first time anything has happened.

so anyway, i was driving down the road and about 35mph, it shifted into third and i rolled into the gas. everything is ok til about ~16lbs, (the scanmaster recorded zero knock this whole time) then a POP and my boost goes to zero for a split second, i shat a brick. so i stop and take off again, this time no boost, just normal accel. and the thing drives just fine. idle is fine, accel is fine, unless under heavy boost.

on the way back i took off and held a steady 10lbs with no problems. so i stop and roll onto the gas again and long about ~16-17lbs, POP! but still no knock and no other drivability issues.

i'm running straight 94 sunoco. no knock was recorded at all. i didn't stay on the throttle after the first pop each time to see what happened afterwards, like more pops, because i was too scared of screwing something up even more.

i absolutely hate working on cars based on 'it sounds funny' but please what does this sound like to you? i am at school right now and the car is at home, so i wont be able to look at anything for about a week but i'd really appreciate you guys' input and opinions.

i'm praying its something simple but i'm leaning towards a headgasket. but to me it makes no sense because of the negligible mods and NO KNOCK!!! anyway, thanks in advance for all the help,
Talon

 

[6underpsi]

Check the boots and clamps on the intercooler and on the up pipe. A loose clamp or bad hose could cause this.

 

[MY T FAST]

sorry, forgot to mention up there, i did a real quick inspection of those, but i was late already for dinner and they were freakin hot!

i didn't take them off but they all seemed ok, i'm gonna check them better next time i'm home. that crossed my mind too, maybe a split in an innercooler hose or something but again i was pressed for time.
thanks,
Talon

 

[GNVYUS 1]

While in park and the cars totally warm post up all the Scanmaster info for us.

What is the fuel pressure with the line off?

What spark plugs and the gap?

How old is the coil and module?

Make sure the car doesn't have any boost leaks and that the line is securely on the fuel pressure reg. I'd be curious to know if the fuel pressure is rising 1:1 at WOT.

I'd drop the boost to 14psi and go from there until it's figured out.

 

[WLDWST]

My money is on a loose hose clamp like 6underpsi posted. Been there. My car wouldn't make more than 15 pounds before it would pop. It drove me nuts looking for it. It turned out that the hose clamp on my lower intercooler hose was just tight enough to pass visual inspection. As a matter of fact it even appeared tight. Make sure the clamp is far enough over the metal pipe and the clamp is tight. Check them all closely. You will find it. It might be something else, but this would be the first place I would look.

 

[MY T FAST]

i thought about the hose clamp too, but why would it just start doing that out of the blue? the clamp just loosened on its own? i didn't change anything, and i'd put about 60 miles on the car earlier that day without a hitch.

stock fpr
plugs gapped to .030
coil and module are stock to my knowledge, (car has 55xxx miles)
i'll post the scanmaster info asap.
thanks,
Talon

 

[WLDWST]

i thought about the hose clamp too, but why would it just start doing that out of the blue? the clamp just loosened on its own? i didn't change anything, and i'd put about 60 miles on the car earlier that day without a hitch.

stock fpr
plugs gapped to .030
coil and module are stock to my knowledge, (car has 55xxx miles)
i'll post the scanmaster info asap.
thanks,
Talon

There is no logic here. Sometimes a clamp can loosen from constant temperature changes etc. Like I said, it may be something else, but it would be the first thing I would check.

 

[MikeL.]

Mine did the same thing at 57,xxx.....wiped the #3 lobe on the stock cam....

 

[MY T FAST]

FatGnat,
i still have stock valvesprings. did you at the time?

how did you figure out that you wiped a lobe.
wouldn't drivability throughout the rev range be affected if a cam lobe has been wiped?
thanks,
Talon

 

[MikeL.]

yup, my combo at the time was a stock motor, adj fpr, 36lb injectors, TA-49, and a 3 in DP, everything else bone stock. No matter if I rolled into the throttle or flooring it, the 'pop' would always happen around 13 or so pound of boost.

I tested or replaced all the easy stuff first to eliminate possibilities, check MAF, swap coil pack and module, plugs, higher octane gas, swapping chips, checking grounds, vac leaks...you name it. Nothing helped.

I eventually pulled the #3 lifter out of the bore and it looked like someone took a carving knife to the bottom of the lifter. Dropped the oil pan and the remnants of the lifter, along with the lobe of the cam, had made a nice silver lining on the bottom of my oil pan

 

[MY T FAST]

nice. and it drove normal up til 13lbs?

well i'm not with the car to check at the moment, but i'm hoping its one of the simple things which i'll check first. thanks.

anybody else? whatcha thinking?
thanks,
Talon

 

[MikeL.]

idled fine, drove fine, pulled real nice up to 13lbs. after that...forget it.

I hope you don't end up with the same issue I had. Good luck.

 

[bmackinnon]

While in park and the cars totally warm post up all the Scanmaster info for us.

What is the fuel pressure with the line off?

What spark plugs and the gap?

How old is the coil and module?

Make sure the car doesn't have any boost leaks and that the line is securely on the fuel pressure reg. I'd be curious to know if the fuel pressure is rising 1:1 at WOT.

I'd drop the boost to 14psi and go from there until it's figured out.

+1 and add the i/c hose checks. My car is stock like yours, and I've had your exact symptoms .... twice..... both times it was the coil pack.... :wink:

 

[MY T FAST]

check for spark? or replace it with a known good one? whats the best way to do that?

 

[bmackinnon]

Measure resistance w/ an ohmmeter between adjacent towers on each coil (between front and rear terms). Should be 11k - 13k ohms. This test will tell you if it's bad, but is not definitive that it's good... Unless you have a Casper's coilpack/module tester, there's really no way to absolutely test it..... so replace w/ known good is the only other option.... The coil pack itself is not that expensive (~$45 ?)... and with your symptoms, the module is likely fine. If the fuel pressure/hose checks turn out ok, my money's on the coil pack. Also, when/if you check the plugs, gap them at a tight .032".....

Edit.... if you replace the coil pack, use OEM/AC Delco!!!

 

[MY T FAST]

i'll do that. coil packs can be found at autozone? napa? or do i need a vendor?

 

[GullsGN]

How stock is stock? If you still have the original hoses from the intercooler, etc., those were too short from the factory and were a problem. New ones are a tad longer and clamp better.

Steve

 

[MY T FAST]

The guy i bought it from put the kirbans intercooler hoses on it. but i'm going to check those too, again probably won't be able to til sunday.
Thanks,
Talon

 

[kingots2004]

mine did this yesterday at 16lbs of boost so im very curious what you find

 

[MY T FAST]

what kinda mods do you have, king?
my car is at home and i'm not so what have you found? intercooler hoses? vac leak? coil? i'm praying i haven't wiped a lobe...