My 89 350 OZ has been running very well until about two weeks ago when it started to miss or cutout momentarily and run well again. This occurs at cruise speed on the freeway (1700 rpm) or at steady 35 to 40 mph in town cruise. It then began to stumble or sag as I accelerate from a light. Idle got noticably rougher too. I've checked fuel pressure both at idle and while driving through the stumble (47 lbs and steady, rises slightly when the stumble occurs then settles back down)
I've checked fuel flow and got 16oz of gas in 15 seconds
I've removed my gas tank heat shield and wrapped it in an insulating blanket plus lowered the muffler another 1/4" with thoughts that I'm boiling my fuel and creating havoc with the pump. It's been over a 100 degrees for a while here.
Ohmed my pickup coil and it checks out to my Helms manual specs.
Ohmed my external coil and it's good.
Checked my plugs they are all nice light tan.
Filters are clean.
Spark plug wires are perfect
MAF wires and connections are good
All vacuum lines are new.
Coolant and MAT sensor are new.
No exhaust leaks, new cat and catback exhaust
TPS IAC (both just months old) set correctly (.55mv and 475 to 500 rpm minimum air)
I found a vacuum leak with my stock throttle body blade shaft and replaced it with a new GM part.
My EGR and solenoid is new as is my charcoal canister, purge valve, and tank pressure valve. I used my Helms again and these all check out.
16 lbs of vacuum too.

I have Tuner Pro RT and while scanning the INT really works hard trying to maintain a BLM of between 119 to 138. BLM does manage to stay somewhat close to 128 on the average. The INT ranges from lets say 115 to 140 while driving in town stop and go. When the stumble sag cut out whatever happens the INT goes to 145 or higher then settles back down somewhat. The BLM has been at 128 when this occurs and doesn't change.

I'm really confused here group. Everything is checking out as working perfect but yet the scanner shows problems and it runs like caacaa (it's getty progressivly worse).

Help me here if you think I'm on the right path.

1) I'm replacing the burp valve on the fuel tank (looks like a white cup cake) thinking that I might be pulling a tank vacuum ( far stretch as the problem happens very quickly after startup).
2) Purchased an injector balance testor and noid light to see if my injectors (new too 21 lbs) are working correctly.
3) What are the chances that my intake manifold (base) has internal cracks that might cause a vacuum leak or transfer of gases between cylinders? Starting fluid sprayed everywhere and I have no vacuum leaks I can find and I tried multiple times.

I've read and searched, tested, read searched, tested and now I thought it's time to ask. I was going to give this car to my kid but until I get it to run and keep running he couldn't possibly maintain this. It's just too glitchy.
Car is all stock except for a AFPR (pressure set from 43 to 47), K&N filters, TPIS airfoil, MSD blaster coil, and a Crane CompuCam 452 intake 465 exhaust. Duration is 214/220 at .050. Heads are new stock iron l98 and the timing chain is new.
Chip is programmed by me off of a stock bin (AUJM). I've lowered the fan turn on temps (it's hot here) and raised the idle 20 rpms from stock to help with the cam, nothing else was needed. BLMS and INT were fine back them. It's been running on this chip for nearly 9 months without issue. I've got a lot of work into this car. GW

 

I feel your pain. I have been chasing a similar problem for 3 summers now. Most of my issues are around idle and stall out when put in gear and at stop lights (when under load) Bought new injectors and tons of little stuff. Lots of test etc. Thinking ignition now but will need to wait until spring to get into it again. I will watch this post so let us know if you find the issue.

 

I'll see what happens thanks for reading this. Maybe the long post scared everyone off.
My cam is really pretty mild and supposedly requires no computer reprogramming. The vehicle has run decently for some time. I'm beginning to wonder though because the cold idle sounds like a stock car and the exhaust is strong (rich smell I think).

I did check my PCV and while the ball does rattle free and it moves up when I plug the end of the valve with my finger at idle. A new one did seem to improve things some. Didn't take it away though so I'm thinking more on the line of coincidence.

Found that when I increase the base timing a lot. Did it just as an experiment. The situation gets much worse.

 

A little more history
Compression is with 8% of each other
Heads are completely rebuilt
Engine uses no oil or has any leaks
Timing chain, cam, and lifters replaced (did this because chain was sloppy contributing to the rough idle, and it needed more mid and top end).
All TPI components cleaned up-EGR and sensors replaced. Not the knock sensor though but tuner says it's working and detecting knock.
Distributor, ECM, chip, and ESC module replaced.
Any bad connector replaced.
Fuel injectors replaced.
TPS and IAC replaced too.
If it's not an injector or two that went bad, bad signal, or a crack in the manifold. I'm out of ideas and places to test.

 

i had a similar problem. I tinkered around with it for a while. things started to clear up after replacing the fuel pressure regulator and a tune up.

 

Individual - was your FPR an adjustable unit? Mine is and I have had it for probably 7 yrs. The guage adjusts with an allen wrench and the guage pin always bounces back and forth. Probably because the engine is idling terribly but I can't rule out anything considering I've been chasing this problem for so long. (similar to GWW's issues)

 

no, its just a plain jane fpr. the diafram was so messed up that it was sending fuel through the vacuum.

 

I didn't read the whole long post so don't slap me if you have already done this. I would check your plug wires first.
I have had similar problems with plug wires being bad including one I fixed last night for the wife's van.

 

Jetmeck, naah no slapping. Sorry the post is so long. Yes plugs etc are all new. Replaced the vent valve on the fuel tank last night because the old one was allowing the tank to pressurize to much. However that didn't solve anything except for remove the gas smell coming out of the rear tire well. The engine runs fine then starts to idle rougher and rougher until it coughs, surges and settles down. At speed in particular at a steady cruize in OD with convertor locked it does the same thing. When cold or warm in open loop it idles very rich and rocks like a stock car. Closed loop is fine, that is until the cough starts.

 

if you mean your fuel pressure guage is bouncing around all the time you have it hooked up that could be your problem.mine is pretty steady everytime i check it on both the gta and the vette.the pressure doesn't change much at all.it used to before i replaced the fpr's.now there both rock steady and it did hesitate before.now runs good.you might want to try a new regulator.