xzodion

Got to say I love this car, but its tiring to drive me in to an early grave.

So a few weeks ago, car had an issue with shuddering and losing power. If you floored the throttle, it would correct its self. It was beginning to happen more frequently. So we changed the ECM, as this had been a problem years ago and it turned out to be the ECM. Well it wasn’t that. I figured since we had replaced the
Fuel pump
Water pump
opti X2
rad
Over flow tank
Low coolant sensor
Coolant temp sensor
And now the ECM.
We also changed the IAC about three years ago
I figured maybe it was the TPS, hence this original post. We also changed the fuel filter which had a bunch of crud in it.

So the car ran great for the last two plus weeks. Last night we went to swap vehicles in the garage and it didn’t want to start. Then again this morning, hard start had to give it lots of gas.

I have a fuel pressure tester.
What are the pressures suppose to be?

And is there anything else that I’m just over looking?
It always runs 94 plus octane and gets oil changes no more than every three months.

Thanks again

Injuneer

Have you scanned it for codes?

Fuel pressure:

Turn key to "run" (do not start engine) - pump will prime for a few seconds and shut down. Pressure when the pump shuts down should be geater than 40psi. After the pump shuts down the pressure should bleed off very slowly. If it drops quickly, could be several different causes, which we can list if that is your problem.

If you get it running:

At idle, remove the vacuum compensation line from the fuel pressure regulator. Correct pressure is 43.5psi. GM accepts anything from 41-47psi as within spec.

Reattach vacuum line, pressure should drop proportional to intake manifold vacuum. Stock cam will drop the pressure about 8psi below the "no vacuum" pressure.

If you can keep it running, tape the fuel pressure gauge to the outside of the windshield, take it out on the highway, and run it up to 5,000+ RPM, WOT. Pressure should not drop below 40psi.

xzodion

Thanks, we will test the fuel pressure this weekend.
We have scanned for codes but none come up. Now the code reader i have is a entry level very basic one. so maybe, its only scanns every 15 seconds. maybe i should get a better scanner.

I will let you know what the fuel pressure test look like. thanks.

xzodion

so..
i tested the pressure last night.

key on no ignition gauge would spike to about 50 drop to 40 then hold at about 48psi

after primming it stayed solid, didnt leak down even i psi over about a five min period.

while running it ran in spec around 40-45psi.

we took it out for a drive and it ran normal.

stopped at a friends house and used a load test on battery and charging system it passses both.

when to start the car and it would not turn over. put the code reader on, no codes. tried to start the cart again, and after a few cranks it started up.

driving home and the gauges and lights "flickered" in time to the reving at low rpm. almost feels ike a miss or a stumble. driving at low speed and alo of a sudden the rpm jump up thne drop to a almost stall. then drives fine.



so, i have not idea where to even start to look.

I have a good mechanic that i take it to, but were tring to pay for a wedding, really cant affors to spend hundreds on diagnosis on a problem, tnhat only happens some, times.

any ideas og what i can look for would help. thanks

Injuneer

Crazy RPM can be due to problems with the high resolution pulse signal from the optical cam position sensor in the Optispark distributor. That signal will set a code if it is completely lost, but will not if it has just become erratic.

What are you using to pull codes? Does it confirm connection?

Could also be a faulty engine ground or PCM ground.

When the gauges and lights were flickeing, what was the reading on the voltage gauge?

xzodion

i was using a equus 0303, basic reader.

It did show that it had a proper connection, but it will only read active codes.

when the lights flicker the volts gauge goes between 11-14volts.

were are the engine grounds locates? the car is old, and i dont think things like that have ever been checked, or changed..

we did change the optic sensor at the same time we did the opti last year, not that it couldn't be defective.

I am goin g to try a better code reader with live reading and see if anything comes up with that.

xzodion

ok so igot a better code reader, and it still pulled up no codes, it showed some live data, and it all looked good. sorry i tried to record it but i wasnt able to.

Thier is a electrical junction block on the passanger side fender, is thier a grounding point? im gonna disconect all the wires and try cleaning it up. they look a little rusty.

Injuneer

Here's the charging/starting diagram for the 96, courtesy of Shoebox. The red terminal box is strictly a positive distribution point. There is no corresponding ground distribution other than the G100 and G101 points shown on the diagram:

http://shbox.com/1/starter_charging_96.jpg

The factory manual shows:

G100 - Bolted to RH side of engine block, same bolt used for A/C compressor.

G101 - Top of RH shock tower

xzodion

not tring to TTT just wanted to add some more information.

Ok. So for the last week or so, the car has been both running great or stumbling and almost stalling out at low speeds. Yester day, the car started fine, she drove to get gas about five minutes from out house, it stalled as she pulled up to the pump. She filled up and the car started up great. She drove it all the way to work no problems, about 30 minutes away. After work she started it up and let it warm up. ( we have been plugging it in at night to keep it a little warmer, this had been helping) She drove to the gym about 20 minutes away and park. Did her work out for, about a hour. Started the car and it would not start. She had to put the pedal to the floor to get it to start. It ran for a few minutes then stalled out. She got it started let it sit for a few minutes and it drop great all the way home. We plugged it in again over night.
Today she could barely get to work. It keeps dyeing out. Stalling and the rpms were all over the place, fluxuating from about 700 to 1500 rpms when at low idle.

I’m at a loss. We had a similar problem about 7 months and we brought it in to a shop, and they had it for a week, and of course it didn’t do anything but run perfect. They drove it around town, on their errands, they even let it sit on a hoist for 6 hours running just to try and get it to present some sort of problem. After the week, we got it back, and the next day it started doing it again. We talked with the garage and decided that is sounded like maybe a faulty fuel pump, and had the whole assembly changed out. It ran good for about four months. Then the stumbling started again, since then we have changed the fuel filter, tps, ECM, cleaned the mass air flow, ( it was really gummed up) checked the fuel pressure. It checked out fine.
Like I said earlier in this post, we have changed the rad and coolant sensor this year, water pump, thermostat, opti and sensor.

I am really at a loss, we are going to bring it back to the mechanic. I give up. Just wondering if anyone had any ideas, that I could tell the mechanic to check for, maybe lower the amount of time they have it for.

Again, I really appreciate all the help, and ideas you guys have given me and any new ones in the future. Ill post more of what we find out in the next few weeks, and hopefully this will be able to help someone else.

Xzod

Chet1185

I am no expert but this is my opinion.

If the mechanic is not familiar with the lt1 engine, my car would not go there. I know you shouldnt just throw parts at it but from what your describing, my car had some of the same issues. My rpm would jump with key on engine off. Run like crap, always stalling. I would replace the opti. For me thats always what my problem was.

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parkers Z28

Looks like you have two problems. Since your fuel pressure is good the problem could be one of the fuel injectors ??? 2nd issue Maybe the switch ??
I didn't see were you ruled out a bad ground are faulty wire ...Check all cables to starter making sure all connections are tight and the hot wire doesn't have a burnt spot in it from the header

xzodion

****update*****
So after the fiancé was barley about to make it to work, last week. We brought it in to our mechanic. they had it for three days.
Day one; started fine, ran perfect. Drove with no problems at all… was started and idled and driven throughout the day. Hooked up to all their scanners and computers, no codes, not problems.
Day two; again started fine, no hesitation. The temperature had risen a few degrees, so it as not as cold. We had the, part it outside to see if it was a cold temp issue ( the starting anyways) car ran fine, no stumbles hesitations or anything. At this point I was tiring to figure out what the fiancé was doing while driving to be causing these issues. We decided that while it was there, we would have the fuel system and the coolant system flushed, as we also had a low heat problem, was pretty sure it was a blocked heater core. So the fiancé called the shop to get them to do this if they still could find anything wrong. And we were told that they had just come back from yet another test drive, this time it stumbled. Lucky they had their scanners all hook up still, and it showed a problem with the IAC. So they cleaned out the IAC, the MASS and the throttle body its self. They already did a complete diagnostic on the fuel system, and told us there was no point doing a flush at this point. They also found that the thermostat was not opening, causing the low temp throw the heater vents, as well as a plug in the heater core.
So we replace the thermostat, flushed the heater core. We pick up the car on Friday, and have so far had no issues.
Thanks to all the people that gave suggestions. Hopefully this gremlin is dead, but I’m sure there will be another, the joys of loving a older vehicle.

Again thanks

Xzod.