I’ve noticed a “stumbling” or a hesitation in my car when I’m cruising or under very light throttle. This occurs in every gear(5spd) but is most noticeable in 2nd-5th gear. When I apply heavier throttle, it goes away. I’m assuming it has something to do with the fuel system. Anybody have any ideas?

 

Check your TPS voltage & fuel pressure; if all that is OK, get a set of used stock injectors out of a Crustang (you can pick those up for like $30 a set) and have Rich at www.cruzinperformance.com do his thing to them, and send them back with GM O-rings on them... I did that to my wife's 4.9 Cadillac, it made an unbelievable difference. It made her sled able to outrun my late little brother's MC SS, instead of being slow like the barge that it is.

 

You think it could be the injectors? Maybe I can just have them cleaned. I replaced the TPS sensor, thinking that was it too, but it still did it. I have an adjustable FPR set at 48psi. The fuel filter is pretty new. Could it be the pump on its way out?

 

It could be the injectors... if it is, I wouldn't waste any money on the Multec crap, put something better in there. The Denso or watever they are that F*rd uses are vastly superior to the Multecs.

Check the other stuff first though.

 

I have the exact same problem, only mine also does it slightly on coast as well. New injectors, Tps,Air control solenoid, fuel pump and filter. Only thing left is the Throttle body. All this stuff was bad at one time or another but you'd think one of them would have fixed it.

 

I was having the same issue recently on my 89 Iroc. Replaced TPS, ran cleaner through injectors, checked vacuum leaks, replaced fuel filter, etc... then one cold night on my way home on an old farm road (no lights or houses nearby) my car went from 60mph to not wanting to run or start. Died and I coasted almost a 1/2 mile. The car would not start. It would turn over but would not fire. Turns out the fuel pump was going out and that is what was causing my hesitation the 3 weeks previous to the stranding.
Local service shop replaced the fuel pump ($500.00 total) and now the hesitation and starting problems are gone.
One thing I hadn't noticed, since it was dying over time and is getting old, was that I no longer really heard the fuel pump turn over when I turned on the ignition. I have had the car for 7 years and realized that when I first got the car I noticed the fuel pump prime all the time. Until I go the new fuel pump I didn't realize that I had not heard that sound very well for the past few years.

So, if all the cheap stuff fails it might be time to check the fuel pump.
My pressure and everything tested fine prior to it failing completely.

 

Yikes...I'd hate to be stranded like that! Its actually been doing it since the Summer with no signs of getting worse. I recently put in an Accel 300+ ignition, changed the TPS and O2 sensors, tuned it up and it still wont go away. Thats why I'm thinking its a fuel thing.

 

If you really suspect the fuel pump, a competent, up-to-date shop can easily check it out for you with an osciloscope and low amp current probe. We've recently gotten more "High tech" at our shop, and we really don't even need to bother with a fuel pressure gauge or volume test to check fuel pumps. We just clamp the current probe over a wire that is carrying current to the fuel pump, and the pattern will tell all! You can get RPM's, amps, rotor/brush wear, etc...from it. Really handy thing to know how to do!

 

Most often, a fuel pump that's about to reach retirement age, will show up at WOT, since that's the conditions of maximum pressure and max fuel delivery simultaneously. As a result, from your description, it doesn't sound like the fuel pump is causing the symptoms.

But in any case, don't just go spending money on randomly replacing parts on the basis of "It might be this" then "Well that didn't fix it, maybe it's that", you will simply go broke fast until you finally get lucky.

You can take the rails and injectors out, hook them all up, and watch them spray if you want to; you can see immediately if any of them are weaker than others that way.

 

I thought that too about a fuel pump. Mine only stumbled between 1200 and 200 rpm. The more pedal I gave it the better it ran. IF I went hard off the line I could barely feel the hesitation. If I eased off the line it jolted like a wild bronco. It only got worse as in the jolting as time went by. Above 2000 rpm there wasn't any hesitation. At WOT it ran great.
Just letting you know another possibility.

Stranded... I spent 1 hour trying to get it to start before someone stopped and took me to their house to use the phone. Then I walked back to the car and waited anothe 3 hours for a tow truck to show. It was a great evening. About 2 hours waiting for the tow truck 2 sheriff's showed up because someone called them and told them they thought someone was setting up a meth lab out in the field. That was great. Luckily for me I used to work security at a bar with one of the sheriffs so he stuck around and we sat in his car, with the heater, until the tow truck showed.

 

Meth lab!

So, the fuel pump fixed it? I can hear mine come on when I turn the key. I thought it strange that it seems ok at WOT too. I'm going to try to get the injectors cleaned first.....I dont think that should be too expensive. Its just so hard to find a trustworthy mechanic around here.

 

No...

RB covered it in his first post. Do some diagnostic work before you throw any parts at the car. Test the fuel pressure and flow. Install a new fuel filter if there are any flow problems, then test it again.

Once you have determined you have adequate fuel delivery, test the injector flow. If you discover an injector problem, forget about the bottled fuel injector cleaners - they don't work anywhere nearly as effectively as an ultrasonic cleaning. If you determine you have an injector problem, start shopping for replacements for your Multecs and send the set to Cruzin' Performance for cleaning and flow-matching. Don't even give it a second thought, since the 'o' rings and clips alone are worth most of the cost.

 

Well, the problem is I'm a little fuel system ignorant. As stated above, I did replace the fuel filter... to no avail.

Therym did mention that the fuel pump was the cause of his problem. I'm not sure that's the case with mine, so I wont running out to replace that right away.

Isn't ther some sort of pressure cleaning that mecahnics do through the fuel rail?

 

Yeah, we do it at our shop for about $70. It's not the caliber of a job you get by sending them out and hving them flow matched and ultrasonically cleaned, but it is a major step up from the can of stuff you put in the tank.

 

If you are looking at the injectors you might want to check out fiveomotorsports.com
They sell factory pulls very cheap. They sell sets of 8 - no miles on them - on Ebay for $150-$170
Again, just some info. I think you should check all the basics, filters and such, before sinking money into anything though. NEver know. Could be a fuel filter.

 

Yes. I had that done to my T/A. A few months later I pulled and sent the injectors to Cruzin' Performance, where Rich discovered at least one was flowing 38% less than most of the others. Did the cleaner work? I don't know, since they injectors may have been worse before the fuel rail flushing, but it certainly didn't solve my problem.

You can test the system pressure at the Schrader valve on the right fuel rail. Pressure on a stock TPI should be 37 PSIG, +/- 3PSIG. If you remove the vacuum line from the pressure regulator or test the pressure with the engine off, the pressure should be 44 PSIG, +/- 3psig.

If you have the correct type pressure tester, it will have a relief valve that can be opened to perform an output flow test. Place the relief hose in an appropriate container and turn on the fuel pump. Minimum output should be ½ pint within fifteen seconds or less.