Hello, and thanks for looking. I've been working on this for the past two weeks, and that comes after a year of several different people trying to figure this out. Here's what I have:

I am looking to buy a '91 Z28 convertible with the 305 TPI under the hood. It had sat for a while and was misfiring and not wanting to start on it's own, so I changed the following (within the last two weeks):

Fuel Pump
Fuel Filter
Fuel Relay
Spark Plugs
Plug Wires (one was burned hard in one area, I think this was causing the misfire)

Basically, the issue we are having now is that the car will start (most of the time) on its own and idle almost fine (a little chug here and there). When we ease into the throttle, it will pick up and go and then eventually drop off, start chugging, and backfire through the intake. If we move the throttle to about 1/4 to 1/2, it will bog down, and sometimes backfire through the intake, for about 3-5 seconds and then start running like a raped ape. It will not, however, "recover" from WOT. It just sits there and jugs like it's barely turning.

I checked the fuel pressure. It stays a pretty constant 40 psi when it's idling. hit the throttle and it moves up to just below 50 psi. I don't see it dropping much at all below 40.

Also, I put the car on a scanner. From what I can see, everything looked normal, but that's from an untrained eye.

Any help here is appreciated. What else should I check?

 

Sometimes the fuel rail can become clogged by old fuel that went bad from sitting. Since you did the forementioned parts i would definately pull the fuel rail and give it a good bath...you will prolly see a white filmy build up in there,may need a new pressure regulator aswell.

 

Make sure the blade shaft on the throttle body doesn't have play in it. I had one tha wore out and in the upper rpm range would bind up and choke the car!

 

It's TPI (Tuned Port Injection)... Not TBI. No throttle body. Or am I mistaken here?

 

We were thinking of taking it to a local shop to have them check out the injectors and fuel system at the top of the motor. Would you still suspect clogged rails or lean towards a bad injector(s) with getting the factory spec 40.5-47 psi of fuel pressure?

 

I would be more inclined to suspect fuel injectors. From what I understand when these cars are left to sit for a while it is really hard on the injectors. This was the case with my car, which sat for 3 years prior to purchase. I would at least test the resistance on them before taking it anywhere.

Also check for vac leaks. Spray around the the intake, runners, plenum, basically any mated surface with some starting fluid while the engine idles. If your rpms increase, there's your leak. Also check for play in play in the throttle blades like J91 suggested. And yes your tpi has a throttle body. Thats the big piece on the front of the plenum with your throttle cable atatched to the side of it (this is how your engine gets air).

Also, what about distributor cap and coil? Did you replace those when you did the plugs and wires? Might be a possibility.

 

Do I have to pull the injectors to test the resistance, our can that be done from the plug? I would like to check everything I can before paying someone to diagnose the problem. Thanks for the guidance.

 

No just pull the electrical connectors off and check resistance through each injector using a multimeter. However, even if the all ohm ok you may still have a clogged or otherwise damaged injector, this was the case with mine. Checking resistance will only detect an electrical problem with them. Considering that it only takes a few minutes though I'd still do it just for the peace of mind.

Check for vac leaks too. Thats also really easy and can save you some headaches.

 

Also, forgot to mention, have you checked the voltage on your throttle position sensor? That could also cause the problems you're describing.

 

Yes, I hooked the car up to a scan tool and the TPS looked as though it was working pretty smoothly.

I went out to the car today (it's sitting at a buddy's shop) and ohms tested the injectors. I could only get the connector off of 7 of them. Four tested between 16 and 17 ohms (16.7 was the highest i think). One tested right at 11 Ohms. One tested at 4.5 Ohms, and another tested at 3.2 or 3.7 Ohms. I checked and rechecked to make sure my reading wasn't wrong.

I got home and promptly ordered a new set of remanned Bosch III's from FuelInjectorConnection.com (199.00 with the gasket kit and fuel rail rings). I'll get them in and post on here if that fixed the issue. There's nothing I hate more than reading through a 3 year old post just to have no resolution.

 

Yes,that would be great. I have a similar problem but not as drastic as yours(Engine performance is just bogged down)

 

It's obvious with these Ohms readings that I have at least 2 if not 3 injectors that are bad. Now that I know this, I've been going over some of the stuff that I've seen as we've tried to diagnose the problems and some if it is a little more clear... Keep in mind that this reply is for my education only, so correct me if I'm wrong on any of this...

The system is bogging down (to the point of almost dying) when we push past a certain point (even slowly) on the throttle (about 1/8 to 1/4 throttle). This is because the bad fuel injectors simply can not keep up with the fuel demands of the motor. I also saw a spike in the fuel pressure when we did this (nearly up to if not over 50 psi)... Any clue on that? Is that normal behavior or because at least two of the injectors are unable to accept the additional fuel?

Also, why would (at lower throttle position) the engine eventually recover? After about 3-5 seconds it picks up and starts running at higher rpms.

Thanks for any clarity on this.

 

shorted injector coils pull hard on the ECm driver, when you increase the injector pulse, you are probably overloading the Drivers in the ECM, also fuel pressure will spike at WOT due to lack of vacuum, which operates the regulator.

 

So, overloading the drivers in the ECM caused the other injectors not to work correctly. Any possible damage to the ECM itself?

Thanks.

 

Wow, UPS tracking can sure as hell make someone OCD. Have to stay focused on work. I just want this puppy to get running again.

 

I just got the injectors into the car. All is well! That thing is running like a raped ape! Now I just gotta bleed the brake lines, change some fluids, etc. Sounds like it needs power steering fluid too. Doesn't surprise me after sitting for a year. Thanks to all for your help.

 

My husband just bought a 91 Z28 305 with 141k miles, (T-top, white with red leather). It's beautiful, but it is doing the same thing - won't go when he presses the gas. It doesn't die on him, starts just fine. He has used fuel injector cleaner with a full tank of gas though; could that cause problems?

I know this is a 3 month old post, but I hope someone will know. We don't have a buddy with a garage to help run tests and we are afraid of getting wallet raped.