So after all the work I have done to my 1985 C4, I am thinking that maybe it does need to be tuned. The new motor is 30 more HP than stock and there are a few mild differences than stock motor. Car has no codes and idles great and has awesome takeoff, but when it comes to just mild cruising, it bucks and stumbles. So my question is, what is my best option for getting this fixed? I am thinking about buying equipment to do this myself. Is this possible to do within reason...$$$? I did read the tune section, but saw the sticky was from 2001 and I am sure things may have changed since then. Thanks

 

Purchase a Email Power Tune from lt1pcmtuning.com. You get 3 tunes for $85 dollars. Talk to Solomon and he will set you up with the things you need to get started. It cost me around $250-$300 for basic tuning equipment.

 

Give us a list of the modification parts you have added. If a new cam was not one of these, the stock chip should be able to handle your modifications.

This sounds more like a fuel problem. Suggest you get a fuel pressure gauge and hook it up. Then tape the gauge to your windshield so you can read the pressure. Then take it out for a run and watch were the pressure is when your cruising.

 

Daniel, Will this equipment work on other year vettes? I also have an 88.
Thanks

 

Hey John, basically I dropped in a Jegs crate motor. I also replaced injectors. As far as injectors, the vendor I bought them from told me they are stock for my car, but they are 21lb and I was told by a forum member that they should be 24lb. Dont know if this could be my problem. I did notice that when I first start the car it doesnt stumble for like the first 5 minutes..... is it getting more fuel at cold start up? I dont know.

 

Yes. I sent my computer out to moates.net to have their adapter soldered in to my ECM. I don't know about how an adapter for an '88 computer would be. The only thing that would be interchangeable is the burner and the ALDL cable and the chips. The adapters have to be soldered in to the computer.

When cold, fuel takes longer to atomize, so you are supposed to run richer at colder temps.

What's probably happening is when you are warmed up, or after the first five minutes, the stumbling is because you are running the wrong injectors. Your injector off sets are designed for 24 lb injectors, and your MAF tables are also designed for that.

So, in closed loop, the ECM will correct, or try to correct the AFR, when you give it gas, the ECM does not have enough time to respond and you get a stumble. This is probably because of one of the following reasons: When you have a higher flow rate, you don't need as long of an injector pulse, when you have a lower flow rate, you need a longer injector pulse. So you are running on a short injector pulse on a low flow rate, meaning you in theory should be running lean. As the ECM corrects for this under a stable condition, as soon as you move out of that range, the ECM is not expecting that change. So, you could be stumbling because of a misfire, or stumbling because of spark knock with timing retard.

Now you can see that when you are cold for the first five minutes it doesn't do this, well that is because you have longer injector pulses. This would occur because you are already rich, so as it leans out when you give it gas, you are still within the okay zone in terms of throttle response and performance.

That's what I think your main problem is coming from. But since you also have a different camshaft in there I think that is also related.

 

Last night I hooked up fuel pressure gauge and I think pressure is fine. It reads 35psi and 40 with the vacuum line off of regulator. I power braked the motor at slight throttle with the car stumbling and the pressure was solid with no drop. So next step is to look into the 21lb injectors I just installed. Forum member mentioned that the injectors should be 24lb. I would think that it would effect WOT with fuel starvation. Any comments on injector knowledge would be welcomed.
Just to recap: idles like a dream and accelerates nice..... but when cruising ..... be prepared for some turbulance folks.
Thanks

 

The 1985 17085050 fuelrail came with 23.8 lb injectors rated at 43.5 psi (3 bar), but installed at 39 psi (2.7 bar) which equaled right at 22.4. By installing a 21 lb injector rated at 43.5 psi at 39 psi it is now really lean. Either replace injectors with 24's, or put an adjustable regulator cap on and turn pressure up to 47 to 48 psi to get the equal 22.4 it is expecting. The later will produce a better spray pattern.

If you've cammed the car you need to find a competent tuner that can provide what you need. 85 tuners are far and few, so be careful. Better would be to update to the 86 thru 89 7165 for faster processor speed and more options, but for some this is a challenge.

Good luck!

 

Thanks, I was thinking about purchasing the 24lb's, but seems like the adjustable regulator would be the easier of the two. Is there a chance of putting too much pressure to the regulator where the diaphragm may rupture or will the 47psi be within the parameters. Also, where can I find a good quality adjustable regulator and I am assuming I should replace the diaphragm also?

 

I've been running a factory 17085050 regulator with Bosch 280-150-561's at 50 psi for 13 years without any issues. I'm originally from California where 10% ethanol has been in use during this time, and my engine has blown up, or have my injectors gone bad. Pretty amazing considering all the posts I see here about how evil things are. My mother in law is still alive though. You think these conspiracy theorists could help a guy out now and then?

Honestly, that good ole made in America diaphragm is superior to any foreign replacement, so you should be good to go!

Oh, almost forgot, I like the www.tpis.com adjustable regulator cap myself.

 

look at replacing the 21 pound injectors with 24's. You should check the specifications on the crate motor's cam and compare it to the stock 85.

85 and 86 cams are the same by my records (vader86)
202/206 @050
Lift with 1.5 R 403/415
Lobe Separation 114.5


If your cam is much different, a tune may be required.

 

I contacted a tuner on forum and according to the specs of the motor, he didnt think there was enough diffence in the crate motor compared to the stock motor to need a tune.

 

Give us the specs on the crate motor please.

If they are similar, its likely not to need much, but between that and the injectors it may warrant a tweak to get it to run right all the time.

 

4-bolt main with 2-piece rear main seal
Cast iron crankshaft
LT1/LT4 powdered metal connecting rods
Cast pistons (dished)
Hydraulic flat tappet cam (.383''/.401'' Lift, 194 / 202 Duration @ .050'', and 112° lobe separation)
Cast iron cylinder heads with 76cc chambers
8.5:1 compression ratio
Block is compatible with left or right side oil dipstick (Requires Plug for Unused Side - Plugs are Included)
4-quart oil pan
Timing tabs for a 6-3/4'' or an 8'' balancer
Recommended spark plug: Accel #111-0574-4 or any spark plug with 14mm .460'' reach, 5/8'' hex, tapered seat, projected tip

 

I concur it is not required with this crate engine. Get your fuel issue solved and your driveability should return.

 

Thanks all. Its driving me nuts that I cant find a local shop here to buy the regulor. I was hoping to get this worked out this weekend.

 

You wont be able to find a store that has it. I think only one company makes it but I may be wrong on that.

It's an aftermarket part that not a lot of people need. It comes with a Miniram Intake Manifold too.

You can get one here. https://tpis.com/parts/view/16

 

Ok, so I installed the adjustable fuel regulator. Got the pressure up to about 45lbs and took her out for a short drive. Car is still running a little rough in the cruising zone and the check engine light came on. Got the car home and pulled the codes........ 42 and 43. I have already replaced the knock sensor and plugs, wires, cap rotor, module.

 

Was the 45 psi without any vacuum attached?

Your codes may be researched here:

http://www.chevythunder.com/Flow%20c...roubleshooting Flow Chart Pages

 

45lbs was with vacuum hooked up. I didnt have the codes before pressure adjustment. I actually had them right after engine install , I then replaced knock sensor and some ignition parts and re-timed and codes cleared. Driving me nuts.