3400 with no idle

[z24turbo93]

i just put in a 3400 that is somewhat built up. I have new 30 lbs injectors, 30lbs chip, brand new 99 fpr, 1 yr old walbro 255lph fuel pump. Now when i turn the ignition on it has 44 psi or fuel pressure and holds. When i start it the fuel pressure goes down a little, but i need to rev it to keep it from dieing. If i clamp off the fuel return line i get 70psi of fuel pressure and the car will idle just fine. As soon as you take the clamp off it dies. Anybody know what could be wrong with it???

I don't think it is the fpr since i have put 2 fpr on there and they both don't improve anything.

[z24turbo93]

this may not be a fuel problem after all. I can unplug the iac and it will idle at 2500 rpm's okay. i have tried two iac's one that is known to work and there is still no improvement.

[prophiseer]

To me, it sounds like the PCM needs to learn the idle curve with the MAF. Hold the idle up right below 1k for about a minute and slowly back off the throttle, if it tries to die, open the throttle back up, and do so until you can take your hand off the throttle without the motor stalling. Once it's holding an idle, it'd be a wise idea to go drive it a bit before cutting it out.

[z24turbo93]

The 3400 is in a cavalier, i believe i don't have a maf. I went out a few minutes ago to check some things and now when i unplug the iac it doesn't hold idle. It dies out the same way as it does when it is plugged in.

Also there is now way to hold the ilde at just below 1k rpm it simply won't run unless you are higher up in the rpm.

[prophiseer]

a PCM controled motor should have a hard if not impossible time holding idle with out the IAC.

What year Cavi is it and what PCM are you using? I'm assuming the 3400 is year '99+?

[z24turbo93]

It is a 93 cavalier. With the iac unplugged it should hold a very high but steady idle so i have been told from a couple of sources.

[prophiseer]

yes, it could. depends on the motor and the PCM. Most motors arent equipped with the sensors and programming to work around it with a 'default idle
a 3400 is when run by a PCM that's programmed for it by use of the MAP and the MAF (known as speed density method)

Wait, though, read your first post again and 'fore this goes awry . . .

What all has been done to the motor?
What are your injectors rated at?
Do you have an adjustable FPR?

Just outta curiosity, what happens when you unplug the vacuum line to the fpr plug it (small 6mm bolt works), and then crank the motor?

[z24turbo93]

The motor is a 2000 venture motor, complete rebuild. It has forged pistons 9:1 compression i believe and block is bored 30 over. It has a crane 260h cam. Yes that is a hydraulic flat tappet cam in a 3400 block. The heads are the same but they have ls6 springs. The injectors are 30 lbs injectors, with a 30lbs chip, the fpr is from a 99.

This is a boosted motor thats why i have bigger injectors

[prophiseer]

yeah, I was heading down the wrong path, sorry!

Alright, then, you say that if you block off the fuel return line the motor will run just fine w/ 70psi? It dawned on me that those 30# injectors need some decent pressure behind them to keep things going. The stock FPR on the '99 3400 is only rated about 40-44 psi at WOT and idle psi is usually about 32-37. I don't think that's enough for 6 30# injectors at idle, the pressure bleed off from each injector spraying would be dropping the fuel press down too far.

Try disconnecting the vacuum line to the FPR and pluging it, then try running the motor. If it seems to run just fine, try grabbing a FPR off an '01+ 3400 (rated something like 50psi+ WOT). If fitment iis the same, a FPR off a 3500 would prob. be a better bet, or you could get an adjustable unit (a few users here know how to make one a lot cheaper than you can buy one).

Hope this helps a lot more than before

[z24turbo93]

I hate to keep giving you the yeah buts since atleast your trying to help. But the person i got the chip from has the 99 fpr with 30 lbs injectors and it isn't a problem. The injectors are multi tec 1's and not 2's so they don't need extra pressure to pop off. Also even if i did put a 2000 fpr in that would run about 44 at idle and it won't even idle with 44. anything lower than 70 psi at idle and it runs like shit. If i have the wrong map sensor in what would that do. Say a one bar map sensor and the computer is set up for 2 bar? I will have a computer on this thing tonight to check it out further, maybe some datalogs if things work out good

[prophiseer]

I take it when talking about a chip, you're meaning an EPROM? So your computer is OBDI, correct? Or are you using an OBDII PCM with some kind of performance module (i.e. Jet module, etc.)?

I don't really think an incorrect MAP would cause this, as long as the PCM can 'interpret' the signal, it might run a bit rough but should still somewhat run at idle with an incorrect MAP.

Well, if you're defi getting fuel to the injectors . . . I'd want to go back to the PCM not realizing how much air it's getting. This is getting me stumped without being able to stand in front of it

[bszopi]

Not sure how you can say it would run correct with the wrong MAP? The MAP tables would be set up for half the regular values, therefore at idle, it would be sending half the actual vacuum to the ECM to determine fueling. I just don't see an engine wanting to idle like that very well.

[prophiseer]

I don't really think an incorrect MAP would cause this, as long as the PCM can 'interpret' the signal, it might run a bit rough but should still somewhat run at idle with an incorrect MAP.

Never said it'd run correctly with an incorrect MAP, just poorly

[z24turbo93]

I checked the map sensor and it checks out to be the 2 bar. Just out of curiosity what would too short of pushrods do to the idle?? I am stumped too i am going to get some datalogs of it very soon and see if someone can determine anything from that.

[farmerz24]

is it possible for the crane 260 to work with roller lifters if it is designed for flat tappet? or did you do something else to get 'round this?

If not maybe thats your problem. does it lope at idle?