91 Z28 305 TPI 700R4 Ok, I'm getting ready to sell my car and I need to fix a couple things first. My main concern is this backfiring issue I have been having with it. I bought the car out of Ohio a couple of summers ago and don't remember ever having this problem the first summer. I then parked the car and let it set for the winter, indoor stored. I replaced the radiator and brought it back out form hibernation this year. On my way to get the plates transfered over, I realized that the engine light was on and the car was acting a little funny. It kind of acts like it wants to lunge forward very slightly while just cruising. This can happen at 60mph, 45 mph, etc. Doesn't really matter on the speed too much. The faster you go, the less drastic it seems to be though. Anyways, the car will free-rev without backfiring, but once you are doing about 70 or so, it can let off a freakin' shotgun of a backfire if you let off the gas and let it whind down. If you are doing about 80 on the expressway, it will backfire basically every time you let off the gas for more than a couple seconds. Very loud! The only other time I can make it backfire is if you really get on it, then let off quickly. Also, when the car is cold and idling, it give this very faint pop pop pop...pop pop pop...pop pop pop. Really weird. It's almost like a very small backfire. Once it runs for a couple minutes though, it goes away. It could just be the crappy muffler though. I know what your thinking, just check and see what the darn engine light is comming on for with a reader. Yeah...uh....for some reason my dad's snap-on reader won't power on when I plug it into the camaro. Weird. It works with every other car I've ever used it on, but for some reason, the camaro won't give it any juice. So, the only lead I got is that the guy who had it before me cut off the cat, and put a really crappy performance muffler on it. I did some searching, but found that removing a catalytic converter won't give you an engine light. However, as I examined under the car a little more, I found that there is a small pipe, about 3/4 inch in diameter that runs along side of the exhaust pipe and is crimped off just before where the cat used to be. Any suggestions??? Besides...figure out why your snap-on reader isn't getting power, and then check the engine light. Thanks, Kevin.

Forget the snap-on reader and use a paperclip.

I'm assuming that was some sort of joke...I just didn't fully understand it, I guess.

you can use a paper clip to read the codes, you bend it in a U shape and insert it in 2 terminals of the diag port, the light will flash according to a code. codes are listed in a Chiltons manual normaly, there should be a section in the tech articles in here about it also, i would find it but dont have time right now, wish i could help more
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PS, my car did that when my collecter gasket on the exhaust got a leak in it, every time i let off after getting on it it would pop pop pop, maybe an exhaust leak?!?!

Oh, wow. I guess it wasn't a joke. I have never heard of something like this before. I will check it out under the tech articles.

I take it that I am not the first to have troubles like this with the code readers...Suppose the harness just isn't getting power for some reason though, then I would assume that the paper clip won't work either. I will try it out anyways.

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P.S. My car does have a small leak somewhere. I just haven't pin-pointed it yet...Can this really cause such a problem???

OK, first I want to say thanks for the paper clip suggestion. I have never heard of this type of thing before and it worked great! Here is what I came up with is I understood the tech article fully.

I got flash flash flash.....flash flash. This happened three times, of course after the initial 1-2 1-2 flashing.

Then I got flash flash flash flash.....flash flash. This then happened three times. And was followed by the initial 1-2 1-2 flashing again. It then repeats itself over and over.

So.....I am assuming that means codes 32 and 42 which is............

Code #32. exhaust gas recirculation valve diagnostic switch OR Fault in electronic vacuum regulator valve.

Code #42. Fault at electronic spark timing circuit OR Fault at direct ignition system OR Fault at fuel cutoff relay circuit

I am going to do some searching but just thought I would say thanks for the help and keep you guys updated.

Had the exact same situation a few years back. Backfiring after letting off the gas and the code 32 for EGR failure. In the end, it was an injector. Replaced the whole set with Accel 24lb/hr and all my troubles went away. Still have the same EGR valve. Also it turns out that I got lucky with Accel injectors. I hear now that they aren't very reliable.

I would say with full confidence that this is an injector or 2 going south on you.

No other suggestions???

You've got a leaky injector.

see, told ya

Tough to replace a leaky injector? I thought it was a vacuum line that I found disconnected. Turns out it's still doing it. So looks like I have to somehow test the injectors. Anybody know how I can do this???

TTT please.

It shouldn't be difficult to change an injector, i don't know a ton about the TPI units, i know that for carbuated it just takes a small screw drives, pop them out, pop the new ones in, screw it down. but then again thats on an old engine. but it shouldn't be to hard.

It will take an hour or so to replace TPI injectors.

You gotta remove the throttle body, plenum and runners then take of the fuel rails which is what the injectors are connected to.

why dont you try from fuel system cleaner first, they are like $5 and run it in a half tank of gas, if that doesnt help then replace injectors

you guys are on the right track but i think you are missing the obvious. The IAC has two functions on a tpi.
1. it lets air in the engine so it will idle. The butterfly in the TB should be closed and the IAC lets air in the engine so it will have air to mix with the fuel on idle.
2. On decelleration it acts like a dashpot. Which means when the butterfly slams shut the engine chokes because his IAC is not opening to get air to mix with the fuel. so liquid fuel gets in the hot cylinder and ignites causing the backfire. On a carb a bad dashpot would do the same thing. The carb dashpot lets the plates close slowly so air could get in. Same principle here only its electronic and is controlled by the TPS switch that must be properly adjusted. .54 volts at idle. so, check both of them and i bet you will be fine. I hope

So, I checked the ohms on the injectors. I did this by poking the wires on one injector with an ohm reader when the car was off. It read 133 point something. I divided that by eight considering they are all connected and came up with somewhere close to 16.25. I know they are supposed to be right around 16.4. Which still tells me nothing because they could all be good and one might be absolute crap. I am going to check them individually this Sunday by disconnecting them. I would like to try what IROCUROC said first though. Im not familiar with all of the terms yet. What is the IAC and what does it stand for? Where can I find this on the car? I have a voltage reader so if you can just be a little more specific for me I can check that. Thanks again for all the help.

The injectors should be shut down on decelleration.

the iac is the idle air control. it is located right under the throttle body. It has a little servo motor that moves a pintle up and down. I do not know if the injectors shut off on decell. but i do know that a bad iac will cause the backfire popping sound.. nothing to loose by checking the free stuff.. also ohm the injectors 1 at a time disconected

Thanks for the input. I am going to check the injectors and the IAC tomorrow probably. I will let you know what I find out. I am supposed to check the injectors' ohms while the car is off, correct? I got the 133 ohm readin when the car was off, but when I started it I got all zeros. Therefore, I am assuming that I need to check them when the car is off. This seemed odd to me because I assumed the opposite at first, so I just want to touch base on that.

the injectors have to be checked individually unhooked from the system. the leads of the ohm meter go one on each terminal in the injector plug. the way you did will not tell you if the injector is any good

Now I'm glad I didn't sell the car to somebody. I would have felt terrible when I got the call back on how F*&#ed this thing was. I checked the injectors. The readings were...

Back Of Car
16.5 9.4
8.2 5.8
15.6 5.1
17.8 16.9

Front Of Car

So I am assuming that I will be fine with just replacing the 4 worst injectors. I am also assuming that this means those injectors are in fact complete trash and cannot be fixed by running a high concentrate of injector cleaner through the car. What I am asking is...does this mean they are mechanically junk or are they just clogged? I don't think an ohm test can tell me that they are clogged, but I just want to be 100 percent sure.

Also, on top of that, if I checked the voltage on the IAC correctly, then I am way off on that as well. I got 11.8 Volts. First, I just want to make sure I know that I tested the right thing. It was located right underneith the throttle body. When I took off my intake, I could see that it allows air to pass into the throttle body. I did start the car and let it idle without the intake on. I wouldn't thin kthat this would matter, but I could be wrong. The harness that plugged into what I tested had four wires going into it. I randomly checked the combination of all four wire with the voltage meter. No matter which two I tested at the same time, I always got the same reading. 11.6-11.8 zone. Did I do this correctly? And if I did, does this mean that the IAC needs to just possibly be cleaned, replaced, or does it simply mean that I need to adjust it. I already searched for "checking IAC voltage" and found nothing that helped. Already Found the tech artice on adjustment. Thanks for the help.

Kevin

P.S. It's a beautiful car. Needs a headliner, and the above stated. $3,500 takes it. At this point, I'm ready to lose my *** on this thing.

dah you need injectors.. call FIC you can get a whole set of injectors for about 130. putting these in is a good selling point. everone buying these is afraid of the injector job.. if you can show you did it right it will help you sell it.. its all apart to replace 4, get them all done
www.fuelinjectorconnection.com

Holy duece! Only $130??? I was told that they were 80 bucks a pop. My big concern is the cash but 130 for a whole set is a freakin' steal.

How about the IAC thing? Any ideas on that? Did I check the voltage correctly or do I got my head up my arse?