z28 Dad

My daughter's 91 5.0 z28 is driving me mad. It started as just stalling when hot, around 200 degrees. I took it to a shop, they said bad TPS and MAP and bad plug wires. I took it home and replaced the TPS and MAP. The TPS was about perfect at .545 idle and 4.1 WOT. I also replaced the IAC, I didn't adjust anything, I just put it in, the car idles great before it's driven so I think it is okay. I replaced the plugs with Bosch platinum plus 2s, the wires with 8.8 mm MSD super conductors, the coil with a MSD Blaster, the cap and rotor with performance Accel units, the fuel filter and all of the rubber vacuum lines just for good measure. Before changing all of this stuff the car stalled after being driven a bit but never hunted. Now after it begins stalling it hunts. I thought maybe the stalling was the torque convertor so I unplgged the harness at the tranny, no change at all.
This may be important, when under the car I saw a stainless looking line from the cat up into the engine compartment, I kind of lose it under there but it looks like the EGR line. This line has two sharp bends in it where the line goes vertical that look like they would almost completely block flow. Is this the design? or have they been damaged and perhaps are causing the problem.

z28 Dad

Forgot to mention, I also replaced the ignition control module that's under the cap. That made no change to anything either.

bigj350

iTrader: (1)

That hooks into the AIR tubes on the exhaust manifold. I think it recirculates unburnt gasses via the smog pump. I might be wrong but I dont think that will cause any problems. Mine is capped off at the cat and Ive had no problems, but then I also dont have the AIR tubes either. However the cat could be getting clogged. I think that will cause stalling.

PurpleHaze92

iTrader: (2)

do the symptoms occur if you unplug the o2 sensor?

z28 Dad

I haven't unplugged it. I'll try that ASAP and report back.
Thanks guys,
Kevin

westcovinadodge

I have a 91 z28, I went through all that stuff a few years ago, seemed like an ignition problem, idle hunt and stall.
check your injectors, ohm them and I bet you will find one or two bad on the o2 sensor side.
after you find the one or two bad ones, change them all. I know, about $100 each, they all fail about the same time so save some money on gaskets, you will change them all anyway.

bigj350

iTrader: (1)

Is the SES light coming on at all? Try hooking a vacuum tester up to it.

z28 Dad

Thanks for the help. I'll try these things out this evening. The SES light is not coming on, it works... it's just not coming on. Do I have to remove the Injectors to test them? how do I do that?

z28 Dad

This is very basic I know but can you guys tell me where the ALDL is? I need to reset the IAC I guess.

QuickStyle

the aldl is located under the drivers side dash about where the brake pedal is, you will have to lay on your back on the floor to see it. by the way, im right here in bakersfield with you, what part of town? if you have time today, you could swing by and i could take a look at it for you, im pretty good at diagnosing these cars.

z28 Dad

I'm in the North West near Hageman and Patton. Thank you very much. I'd like to bring it by. I'll PM you my number.

z28 Dad

Reset the IAC and TPS. Disconnected the O2 sensor and it totally changed. It idled high, 1100 rpm in P or N and about 800 in D but did not hunt or stall any more. I got pretty geeked up so bought a new O2 sensor. Installed that and it ran just Like it did when disconnected, high idle, no stalling or hunting. Reset the IAC and TPS again. Hunts and stalls just like I did nothing at all.

I still haven't check the injectors. I'll do a search to try to learn how. In the meantime please keep the hints coming.

Thank you Quick for your help. I may call on Sunday.

z28 Dad

OHM tested the injectors. # 1 and 2 both 8.8, #3 10.2, the rest all 16.0 to 16.3. Is this enough to cause problems? What injectors should I buy if any?

QuickStyle

yep those ohm readings are too low. they all need to be 16-17 ohms. if i remember correctly you said you had a 5.0L so you would need 19lb injectors, i recommend the ford injectors, and so does 95% of everyone on this board. i have a set of ford 19lb's that i just got back from rich at cruizin performance that i dont need anymore if you are interested. give me a call sometime today, and we can figure something out, i'll be outside working on my car for the next 6 hours or so, so its no bother.

PurpleHaze92

iTrader: (2)

Did you solve your Issue?

QuickStyle

yea i helped him out on this. he replaced the o2 sensor, it didn't help, although it probably needed to be replaced anyway. we put some 19lb for svo injectors in, and it solved the problem. half his old injectors were ohming in the 7-8 range. we also replaced the fuel pressure regulator cause it would not hold pressure over 48ish, so im sure that had something to do with the driveability issues.

PurpleHaze92

iTrader: (2)

Good! Glad to hear that yall got it fixed. Oddly enough that is exactly what fixed the same issue that I had awhile ago. I used 19lb SVO injectors too LOL...

Elephantismo

I would say the primary problem was the fuel pressure regulator. When mine went it did the same thing. I didn't know it was the regulator but i knew when i'd pressure down the fuel line, the problem would correct itself temporarily. The spring in mine was totally gone by the time i figured out it was the regulator and not any of the three fuel pumps that was the primary cause. Parts house giving me a TBI fuel pump the first time didn't help, since low fuel pressure was the overall effect and the weak pump exacerbated it.

Replacing the injectors prolly helped a lot for the driveability no? I'm about to do that job on mine, along with some other tuning. 305 to 350 swap just wrapped up and it's acting lean.

QuickStyle

the primary problem was the injectors, half were ohming out at below 8. the fuel pressure was steady at 48psi and would not go above that, so that would have no effect on the idle and it ran fine when it was cold which = bad injectors. but by replacing the fpr it helped the flow rating be more consistent as the vacuum changed, thats what its for. but not his car runs fine.

Elephantismo

Oh, well its not the same issue then.
My FPR failed to produce very similar effects. Tiny cracks in the diaphragm allowed fuel to come into the vacuum system and effect the idle. Then once every thing was hot, the fpr would open up into the return line from pressure and would not close until the fuel pump was shut off and, as the problem got worse, until the line pressure was bled down.

z28 Dad

Sorry it's taken so long to post again. QuickStyle fixed it. I drove it today for the first time other than driving it home from Quick's place. I wasn't certain then that it was fixed but I am now. I drove it 25 miles to work this morning, and never had a problem. Thanks again Quick.

Elephantismo

Lets hear it for 91 z-28s!

Jay

iTrader: (2)

I have a 91 that's got simmilar issues. Gone through a bunch of things at the mechanic that didn't work. I'll have to test the injectors this weekend.

vette1954

I have a 91 Z28 that I am the second owner. It has 51,000 miles on it and sits most of the time. It is 5.0 and I am now doing the same thing. To find out why is is not running well. I have a Tech 1 scanner and can monitor what is going on. The fuel pressure at the time I turn the key on goes to 55 psi and then drops to ~45 psi. when the car does start the fuel pressure guage in not steady but varies +/- 3 psi. Is this normal ? In addition, the car will not go into closed loop. What is the cause of that ?