Help with my truck

[JoeliusZ28]

Edit: This is a 1998 Vortec 350 L31 engine. It is all stock except for a CAI and dual exhaust.

OK, as many of you know I replaced the leaking intake manifold gaskets on my truck over winter break. Everything is back to normal now, however I have a strange problem - and no one seems to have any idea what it is.

Symptoms:

• I have been getting a Service engine light randomly - however the kicker here is that this started before the gasket change, and it also evaded me the entire christmas break when i had access to a scanner. lately it has been on much more often.
• Vibration at high RPM. It is most noticeable in the 4500-5000 RPM range before a wide-open-throttle shift, and it makes nearly as much noise as my open element air filter vacuuming under said conditions. (pretty loud)
• The truck feels sluggish. The engine feels/sounds healthy as an ox but seems as though something is dragging on it. Throttle response seems lazy, and i feel like i have to press the pedal more. This part might be just in my head, but it seems like it tires to find the higher gear a lot more aggressively than it did before....and just doesnt seem as "ready to go." All that being said, it still comes VERY close to barking second gear the way it always has. Also, i dont think it is a timing issue - the exhaust note sounds the same as at always did, and there is no shake or anything like that.

Im clueless, and it is really bothering me. This truck always put a smile on my face because of how good and strong it ran, but flooring it now feels like im whipping an injured horse.

I didnt have one single leftover part or bolt. everything went back together perfect. I replaced the waterpump with a new one even though it didnt need it (aftermarket pump from murrays).

If i had any extra money id be diagnosing it with an OBD2 cable and HPTuners but im broke. I had both a mechanic drive it, and my dad - and they dont think anything is wrong with it. I couldnt get it to throw the code for the mechanic, but he thought it was just the gas cap.

If anything can think of anything to check i would be really grateful.

[81DaytonaPaceCar]

My first impression is that it would be something unrelated to your previous problems, and probably something simple.

But I'd definately try to see what codes it's throwing.

MAF sensor?

[JoeliusZ28]

I will try to take it up to autozone tomorrow and get it scanned.

What makes you think it would be the MAF sensor? I agree it could very well be unrelated. However, the vibration started at that exact point in time. I noticed it before I had even driven a half mile.

The only moving parts I messed with are the distributor, water pump, and fan. I noticed that the fan blades are unevenly spaced. I am assuming it was engineered to be a balanced item, so it shouldnt matter if i put it on a different way than it was on before... (if that is even possible) correct?

[WMCCjames]

Just because the light is not on doesn't mean there isn't a code......should be stored unless you have disconnected the battery since then...should of had the mechanic scan it regardless. Take it down to Autozone and have it scanned if your in a bind....until you figure out what that code is, you could end up chasing your tail.

[JoeliusZ28]

Just because the light is not on doesn't mean there isn't a code......should be stored unless you have disconnected the battery since then...should of had the mechanic scan it regardless. Take it down to Autozone and have it scanned if your in a bind....until you figure out what that code is, you could end up chasing your tail.

yea i know, that was the problem. i had disconnected the battery to do the gaskets and the scan tool wasnt picking up any codes. I couldnt get it to come back on before i had to leave for school either I'll check it out tomorrow.

I guess to me it seems like something is out of balance somewhere, and the code is something else. but ill see.

[Phantom409]

I would go back and just double check everything you did and make sure you didn't miss getting a bolt tight.

Throtle response could be the MAF like Jeff said.

[Phantom409]

Your not loosing a U-Joint in your drive shaft are you??
I had that a few times, caused a bad vibration.
Also, I learned this from my car:
If the stock drive shaft has a damper in it, that could be bad too.
Can you fell it in the seat of your pants?
4x4??

[JoeliusZ28]

the vibration is RPM related. Im still on the original u-joints but I dont think that is the problem, I know what a bad u-joint feels like.

I double checked everything 50 times... i couldnt have felt any better about how it went back together. I had every bolt labelled and used a tq wrench where needed, etc.

[meissen]

I know it sounds stupid but - EGR valve?

SES light will come on when it gets stuck, it'll make the throttle feel weird, and on the 3.4L when the EGR valve got entirely stuck it was sucking air in like CRAZY (you said yours is making a noise like an open element air filter).

[MP81]

I know it sounds stupid but - EGR valve?

I was thinking that could be an issue.

One never knows!

[JoeliusZ28]

I know it sounds stupid but - EGR valve?

SES light will come on when it gets stuck, it'll make the throttle feel weird, and on the 3.4L when the EGR valve got entirely stuck it was sucking air in like CRAZY (you said yours is making a noise like an open element air filter).

no i was saying it is as loud as that. however that is a possibility... just dont think it is my vibration. I think i have more than one problem here.

[1BADAIR]

egr valve are pretty common on those. normally the light will come and go. The light normally will come one decel after a steady state 45+mph cruise.
another issue is depending on if it was burning coolant too the cats could have been damaged or are bad.
take it to autozone and check for codes

[zmanrc]

A bad/cracked spark plug wire?
The vibration is that cylinder missing at higher RPM's.
If it's cracked inside you won't see anything wrong and it may not miss all the time.
A scan should even tell you which cylinder it is.

[Bad_Z28]

I wonder if any coolant was ran through the engine damaging a O2 sensor causing the engine to run "fat"....

You may just be stuck waiting for that light to flash back on and re-scan....good luck!

[JoeliusZ28]

egr valve are pretty common on those. normally the light will come and go. The light normally will come one decel after a steady state 45+mph cruise.
another issue is depending on if it was burning coolant too the cats could have been damaged or are bad.
take it to autozone and check for codes

im fairly certain the leak was only external based on how the gaskets looked, but that is possible. I am 100% positive that none got into the oil, at least. The cats arent rattling like the 3 others ive had go bad on my vehicles.

EGR could definitely be bad but i dont think it is anything to do with my vibration.... EGR shouldnt even be functioning at 4500+ rpm.

A bad/cracked spark plug wire?
The vibration is that cylinder missing at higher RPM's.
If it's cracked inside you won't see anything wrong and it may not miss all the time.
A scan should even tell you which cylinder it is.

It isnt missfiring. I have a hell of an ear for engines - trust me, its firing on all 8 and its running strong...the power just isnt getting to the wheels. I installed brand new plugs also. I could hear it missfiring ocasionally on the old plugs - which is why i replaced them