Hello,

I'm doing work on a 1991 Z28 305 TPI.
I'm having a little trouble running my engine, as similarly described in this thread from a whiles back: https://www.thirdgen.org/forums/tpi/...ough-idle.html


I figured i'd start a new thread, rather than pull up a zombie.
Anyways, after reading through, I have a few questions:


1) Given that the car idles well for 15 minutes before it degrades, does and how would heat/run time affect the fuel injector's performance?


2) If the fuel injector is indeed damaged, would it really mess up the computer? Why, what is the connection?




Any help is much appreciated. Thanks

 

You need to do some basic troubleshooting. First I'd disconnect the O2 sensor to keep it in open loop to see if it makes it better. I'd also check the fuel pressure both when it's running well and when it's not.

The blowing through the defroster vents is a sign of no vacuum to the heat/ac, so you probably want to check your vacuum hoses too.

 

Silly question: how would i measure the fuel pressure? is that something i'd need to take the car into a shop for? or is there a gauge I can rent from like pep boys.

Thanks

 

You need a gauge. A lot of the parts stores loan tools like that for free. Ask around.

 

I had a similar problem, but my car also has a bigger cam. It turned out that the injectors contributed to part of the problem, the other part was due to the need for a computer tune. The injectors were not leaking, but they are not flowing as well anymore and are very loud.
Get a fuel pressure gauge. Check the fuel pressure at the fuel rail when you first turn the key to the on position with the vacuum port to the fuel pressure regulator disconnected. It should go up to 50 PSI and hold steady. Plug the vacuum port back up and start the car. Fuel pressure should be a little bit lower, around 42 PSI and should stay steady.
If the fuel pressure decays after you turn the key to the ON position and the fuel pump has stopped priming, you could have leaking fuel injectors, or a bad check valve in the fuel pump. If you see any fuel coming out of the vacuum port of the fuel pressure regulator, it would mean that the FPR is toast.
Perform the fuel pressure test also when the car has warmed up.
You should also test the resistance of your injectors when cold and when hot as well.

 

jv9999: awesome, thanks for the help, I will get on that asap.

Saculia: thanks as well. questions though:

1) what does a "computer tune" entail? I assume you don't mean replacing the entire computer. And am I able to take the computer out on my own and have that done somewhere, or is it best to have a shop do it?

2) I've been looking through my service manual for a procedure to check the resistance of the injectors, (i'll continue looking later after work). however, let me ask, how do you do a resistance check? is it as simple as touch the probes to both contacts? should the injector be taken out to do this, or does it not matter? and what kind of resistance am I looking for.



Thanks again!

 

Joe just another follow-up on that sound 'somewhere up front' you hear when the engine is hot will not start (posted elsewhere) ... In another thread the issue is brought up that it might be the sound of fuel returning to the fuel tank through the fuel pressure regulator, or heaven forbid, being sent out the fuel pressure regulator vacuum hose. On your engine you said you could hear the sound ONLY when the engine was hot and would not re-start ... so the fuel pressure regulator looks like a very good thing to investigate.

 

I had similar problem, it runned good while cold, the hotter it got the worse it idled.. turned out to be bad injectors. Measure them hot and see what ohm they got, if they are below 12 they toast, they still can hangup and so if the values are good but. Check your injectors.

Ive tested 4 different injectors, Accel, Ford Racing (Denso), Multec and Bosch III and I can say, Ive never got better idle and throttleresponse then with the Bosch III from FIC, So if you got injector problems, call Jon up and he will help you out. He leaves 3 years warranty on his injectors. So my advice is. Check your fuelpressure, and injectors. seems like they cuts out when they get hot.

 

A computer tune entails that you modify the program that the computer runs. In my case, that means modifying the fuel tables and also spark table s in ways that I can have the right amount of fuel for the right amount of air that is being moved through the engine. If you have not changed injector size or upgraded things like your cam, intake, or heads, you probably don't need one.

To test the resistance of your injectors, you need to unplug the injector connector, and test the resistance across each injector terminal. They should be somewhere between 14 and 16 ohms. This value should not change as the engine gets hot. If the resistance of an injector is below that value, it is probably out, or on its way out. However, this only verifies that the electric coil of the injector is fine. It does not verify whether or not you have a clogged injector or a leaking one. Sometimes when the injectors are getting worn out, they will get very noisy, as was in the case of my car.

 

Theking: through trouble shooting and process of elimination the past few days, i think i've narrowed my problem down to there being bad fuel injectors or (i really don't think so, though) a failing fuel pump (just as you said). i tested resistance of the injectors and HOLY SMOKE! 4 read in the 17-18 ohm range, the other are 2, 3, 8 and lucky 13 ohms. i'm obviously going ot replace the 4, but a few a questions:

1) you mentioned the Bosch injectors. would anything with the computer have to be modfied because the injectors are not factory? (and who's Jon?) lol

2) while replacing the bad 4, would it be worth it to have the good 4 rebuilt? i found a few websites that clean and rebuild the injectors for like $25. does anyone know if this is worth it? or complete BS?

Thanks!

 

I would buy 8 new ones

 

Jon is the owner and founder of Fuel Injector Connection.
He is really nice and really wanna help.

He sells 8 Bosch III injectors that flows 22lbs/hr he leaves 3 years warranty on them, and they are flowmatched, wich means that they flow exactly the same. The price for 8 is : 169,95 so I would recommend buying these ones before any other ones. Never had better injectors in the car.. (as stated before, Ive tested some...)

You need to lower the fuelpressure on a 305 to compensate for the higher flow, the stock 305 are 19lbs...

Link to the injectors : http://fuelinjectorconnection.com/sh...&productId=134

 

ha, that's funny, i was just looking at the site earlier today.

how would i go about lowering the fuel pressure? I assume i'd have to pop in a different/new fuel regulator?

Thanks.

 

Yes you need to change the stock regulator.
Or reburn your prom to the right Inj. Constant. That works better thou. because then you got higher pressre = even better spray pattern..

So you need to think if you wanna buy a new regulator and/or a custom prom.
www.tpiparts.net sells custom proms for 65 dollars.. a regulator is ~same price, I bought a Holly one for 95.95dollar (that one you can adjust with your fingers, no tool needed for adjustment)

So its up to what you wanna do.

 

sorry to revive an old thread, but i just wanted to follow up with some closure:

I bought a set of Boscsh III's from Fuel Injector Connection. Before making my purchase, I spoke with Jon on the phone. VERY knowledgable. Similar to everybody else who has bought them him, I highly suggest them.

Anyways, I bought a set, popped them in, and ta-da! problem solved! In fact, the car runs a lot smoother now, not as shakey.

I officially put my car on the road last week, and have been driving it around. It drives very well, can't complain.

Thanks for everybody's help.