I have had the stock fuel pump for a long time (it is a 96). I am just wondering what the symptoms were for others as it started going out.

Is it a gradual thing (once it hits a certain pressure) or is it an all at once thing?

I always replace my opti when I get a high rpm miss and it always fixes it. I am just afraid one of these times it will be my fuel pump instead of my opti, and I do not want to replace the wrong thing.

With ~400 rwhp I know I am close to the limit, but I am not there yet.

 

Mine started taking longer and longer to start in the morn or after it sat for and hr or more. Replaced it with a walbro 255 and fires right up anytime now.

 

You could pick up a FP gauge to verify it's OK . Usually they fail 3-ways , 1-don't run at all causing no start. 2-flow and pressure starts dropping causing high load miss, hesitation, crappy running. 3- w/engine off rail pressure bleeds off due to bad check valve in pump and causes hard starting after sitting a while. IMO numbers 2 and 3 seem the most common on these forums. With 400 rwhp and 14 year old stock FP you could be getting close to number two. Try taping a gauge to the windshield during a WOT run and see where you are ....

 

Bobdec,
I agree with your statement on the #2 and I really would like to do this. Unfortunately, I cannot do a WOT run anywhere near my home and to get a run like that I usually have to get to the track. I have only driven my car like 10-15 times this year, and each time was at the track. That is the unfortunate part of having offroad pipes I guess.

This is the reason I am replacing my opti (high rpm miss) and it has gotten progressively worse the last 3 times at the track. The last time my MPH was up big time (93.5 in the 1/8) but the high rpm miss was worse than ever as well. anything above 6000 RPMs is a pop pop spit and go...

I have had this exact same problem with probably my last 2 opti's. I already bought a new opti and a new fuel pressure reg. Maybe before I start tearing it apart I will take it to a dyno near me and check the fp though. It has not had troubles starting though, and in 10-20 deg the last 2-3 times I started it that is saying a lot (my diesel truck barely ever starts that quick). SOOO... in other words maybe I should just replace the damn thing and forget about it.

 

You don't have to wind it out through all 6 gears..... just in 1st gear - about 45MPH with your 4.10's. Why is that not possible?

 

It is just very loud and my neighbors around here do not like my car very much. They seem to call the cops every time I take it out. But, police response time around here is ~8 minutes... I can probably go and be back while checking fp in 1st gear! Now I just have to wait for the snow/ice to go away.

 

I could have written this post about my '97 LT1 problem. I have a high rpm miss that comes in at 5000 to 5500 and then smooths out to redline. I have changed plugs, plug wires and then just put in a new GM Opti. I was better but it continued to have a slight stumble noticeable only in higher gears. After about 150 miles it became more evident again. HELP!!!

I checked fuel pressure a few times over the years and it is always has been good at idle. I don't remember for sure but it was in the 45 lbs. range. I remember plugging or unplugging the vacuum line??? I did modify the regulator to make it adjustable.

WHAT DO I DO NEXT? Check fuel pressure on the road as suggested? Change the Opti Spark again? Change the Crank Sensor? Change the fuel pump?

Engine has 5000 miles since rebuild but this has been a monkey on my back since before that. The engine is normally aspirated, no Nitrous and puts down about 350 rwhp.

 

Why would you start chanbgin parts like the crank sensor (used for misfire detion - not to control the engine in any way), the Opti (which you already replaced), etc. without having the slightest idea what the problem is?

You need to check the fuel pressure under load.

Have you had the PCM scanned for codes? With the correct scanner (GM Enhanced Parameters) you can get the misfire count by individual cylinder, which will often point you in the right direction.

 

I will take a pressure gauge and see what kind of fuel pressure my car has under load while driving and report back to this post.

I have read numerous posts on this message board. Many times they have been helpful and then their are numerous posts that lead to a dead end.

For anyone reading this....if you have a problem and get it solved please report back so we can learn from your experience!

Thanks Injuneer for your continued help. Keep up the good work!

 

I have not checked back on this since last season when I noticed the problem. I am changing the oil this weekend and taking her for another drive, maybe one of these days I will check it with a fuel pressure guage... soon hopefully.

 

When mine went out on a stockish '94 Z28 it had a rough idle and ocasionally didn't want to start, a few times when driving it had a big loss in power. Pretty weird.

When it went out on my old beater ('96 Silverado 5.0L 2WD) it simply died as I was pulling up to a stop light.

 

I checked my fuel pressure and it is as follows:
Key On = 40 lbs.
WOP = 45 lbs.
Idle/Cruise/Off Throttle = 35 lbs.

These readings don't indicate that my problem with a high speed miss is related to my fuel pump......

I may need to switch threads unless anyone else can relate this to fuel delivery. I have changed the fuel filter, injectors, ignition wires, spark plugs, opti-spark and added a MSD 6 Plus module (but still running the stock coil since the MSD coil crapped out). It still puts down 300+ hp to the wheels but the graph does show this hiccup at 5500 rpm. Check engine light did appear regularly with a 0300 random cylinder misfire code. Changed #1 cyl plug and wire and made it go away but not completely. It has appeared once in the last 500 miles. Has anyone had a brand new OEM Opti-Spark that was bad?

 

I don't know why any ignition or fuel component would cause a drop at 5500 but be fine both below and above that. If you have a laptop you might try uploading a different tune and see what it does. Actually, after all you've replace already, I would definitely do that.
You could try a new alternator also. I've heard that when they go bad they can make a lot of things act real funny. That probably isn't your problem, but it is something else that's part of the system.
I've also heard from someone who had trouble with a diesel boat engine that there was a brief drop in fuel pressure sometimes but an electronic gauge was too slow to catch it, but he did see it on a direct analog gauge. But then I also don't know why any fuel pump or anything else in the fuel system would cause a drop just at a particular RPM but be fine otherwise.
If you are really confident in your diagnosis that all is well except at a particular RPM, I personally would suspect ignition rather than fuel being the problem.

Good luck man.

 

The good news is that this hiccup won't stop me from doing an Open Track Day later this month! I will be busy with installing new brake pads, a brake bleed, harness belts and Hot Lap Timer and mount up the set of Victoracers that have been on the shelf for too long............I will keep you posted on the hiccup if anything changes.

And so long for now.

 

my blip was on a really cold night at the end of last season and not JUST at 6000rpms, but any rpm ABOVE 6000 rpms. So it would pop, catch up, pop again. I will go to the track this weekend or next. If it is next weekend I will just replace my opti and install my new LCA relocation brackets just to be safe. If I get the bug tomorrow night, I will just see if this opti is still good. Might just end up doing that!