Another Fuel Pressure Question
 

I've read post after post and thread after thread about fuel pressure, egr, weird surging etc... So i have gained a good bit of knowledge about the tests to run and what not. I believe i'm on some what of the right track, just getting a little overwhelmed. I have a 1990 5.7 tbi that idles fine (thanks to a new cts). After it warms to "operating temp" and is put under a load off idle, it stutters. I have noticed some backfiring through the tbi when its put under a heavy load. As i go down the road it seams to have a lack of power, more like something is holding it back. Cruise at 65-70 i can feel hesitation.

-Things done so far-
-New CTS and pig tail.(because it was the easiest least expensive place to start) Did help out some.
-New EGR Delco PN214-5073 Old one was not hooked up to begin with and diaphram would not move. Pulled and was pretty carboned up. Worth a shot eh! New one installed at idle, pulled in diaphram and engine dies. From what i have read this is supposed to happen. Also means passages are not clogged? Was getting SES light with code 32. No longer an issue.
-Pulled IAC valve and pindle was carboned up. Cleaned and did the best i could without removing tbi to clean passage where it seats. WD-40 and Q-tips till they came clean. Still get a little pulsing idle but not as bad.
-Plugs, wires, cap and rotor all new. Less than a year.
-Rented a pressure test kit from Auto Zone(Actron) Installed behind tbi which was a bear to do but figured it out. Did not come with adapter for fuel filter install. Turn key on and needle jumps up and fades away. Key off and try again with same results. Start truck and jumps up to 12.5 psi and is steady. Kill engine and needle drops to 0.

Ok. Heres my question in a question.

Since i have good fuel pressure, could this be the regulator or the split in the hose in the tank. I would think the system should hold pressure when the engine is killed, right? Would the split in the hose cause the system to loose pressure when the engine is killed?

Back to the IAC valve. Everyone iv'e talked to, out of this forum, says thats my problem. They always mess up. Is there a test? Or do they just "mess up".

I think i have covered all the bases. Mabey missed a few. I know its a long post, just trying to be thorough. Thanks for any advice i can get.

Re: Another Fuel Pressure Question
 

You have a steady fuel pressure of 12.5 so look for a problem else where.

As far as the pressure dropping as soon as the pump stops – this is OK.
Some TBI units had a hole in them that bleeds off the pressure as soon as the pump stops, and I have never been able to determine which TBI unit had the holes, without taking them apart.

The IAC controls the IDLE speed and will not cause a stumble when the throttle is open.

The electronic spark control can cause a flat spot or feeling of lack of power. The Knock sensor contains a piezoelectric crystal which causes a voltage to be produced. Loose components on or in the engine can cause the knock sensor to create a voltage and the system may interpret this as pinging. The system retards the timing because it determines it is pinging.

The backfiring makes me think the engine may have a mechanical problem.
The head gaskets do fail on these engines and when the head gasket begins leaking between the center cylinders on either bank it can cause what you describe. These engine often ping when the head gaskets are beginning to fail.

An exhaust lobe on the cam that is getting flat can cause a backfire. When this is happening it is most noticeable when you crack open the throttle hard, when the truck is in park, with the air cleaner off. The ZDDP change in motors oil seems to have made this a more common problem.

Don't overlook an easy fix like a bad distributor cap, with a carbon track, leaking spark to the wrong cylinder, causing the problem.

Re: Another Fuel Pressure Question
 

Hmmm... Totally not what i was expecting. Your mention of spark and electronics does bring up an issue i was having just before all the plugs, wires and such were changed. Iv'e actually gone through 2 dist caps in the last couple of years. The one thats on now has lasted quite a while so far. It seemed as the rotor was rubbing off to the side of center (if that sounds right) under the cap. I was thinking i had it too tight and it wasn't sitting flat on the dist base. The point on the rotor that makes contact with the center of the cap was completely worn flat, as was the cap contact.
Now iv'e read on some of the posts that the distributor can wobble over time on older trucks making it run bad. I'll do some more investigation on that area as to what and where to look for. Hopefully i have an external issue and not an internal issue.
Something else comes to mind. I was looking at timing today and found that the brown wire that you unhook to check timing is already unhooked. Checked timing anyway and was at 0. Will this effect anything if wire is not plugged back in? If so, where? Because i couldn't find the mate to this plug(coming from the right towards the electrical box on fire wall. I did find some wires that had been taped together coming from the harness. Didn't look factory to me.
Once again your service and info are greatly appreciated. Hopefully i have giving a few more bits of info that may help you help me. Thanks again.

Re: Another Fuel Pressure Question
 

You're welcome!

Yes, that will make a big difference.
If it is the set timing connector that is between the distributor and ECM that you found unplugged, then the truck will be stuck at base timing. It will run but lack power. It will idle poorly, because it is at least 12 degrees retarded of where it would be with the system working.

That truck is going to come to life once you get the problems taken care of.

You should check for trouble codes again, because if the set timing connector is unplugged it should set a trouble code 42.

Re: Another Fuel Pressure Question
 

Distributor information

On these distributors the housing/upper bushing area can get so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

Around 1989-1990 the distributor shafts didn't have the groove in the shaft to direct oil "back" down into the engine. Oil will get into the pickup coil and can damage the coil windings. Any of the distributors can do this if they get very worn.

If there is oil coming up the shaft and getting out onto the pickup coil – replace the distributor.

These distributors can also have problems because of loss of magnetism needed for the pickup coil to work.

When you have the distributor cap off wiggle the shaft and see if it moves sideways.

Also check the magnetism by taking a small end wrench and touching it to the reluctor (the round part on the distributor shaft just below the rotor) and make sure you can feel the magnetism as you pull the wrench away.

Re: Another Fuel Pressure Question
 

AH Ha! Now if i can just find out where the other end is. I know its a male/female style plug but like i said i couldn't find the other end. And there is the mystery wires that are taped up. Is the other plug tan/black as well? I was just reading a long thread from back in 09 and saw that you were on line so i figured that i would check if you replied. Uhh! Stalker!:lol:
Cool. I have high hopes now. Thanks again.

Re: Another Fuel Pressure Question
 

For 1988-1993 C/K pickup trucks:
To set the ignition timing you must unplug the set timing connector. The set timing wire, which is a single Tan wire with a black stripe/tracer on it, is located, under the hood, on the main harness that goes from the engine to the firewall, on the passenger side, sometimes underneath the black plastic cover (if it’s still there) that covers the junction block, relays & fuses. It’s near the top usually on the back side of the harness. From the factory there is what looks like black tape around the harness holding the set timing wire to the back of the harness. Doing this will set a code 42 in the computer. Clear the code after you are done checking or setting the timing.


http://67-72chevytrucks.com/vboard/a...1&d=1251120221
Image is from this thread:
http://67-72chevytrucks.com/vboard/s...d.php?t=358853

Re: Another Fuel Pressure Question
 

***NEWS FLASH***
-3STRATS is color blind!!!!!!
Ok the wire in mention that iv'e been looking at is Magenta or Purple. It has a round male plug end. I think its called a weather pack or something with the rubber gasket grommet. It comes out of the middle of the "y" part, or the intersection of the harness that comes from the relay box. Still havent found a mate for this one.

Now on to the good stuff...

I found it! While looking for a mate to the timing wire, or what i thought was the timing wire, i stumbled across this other plug coming from the harness. Low and behold it was brown and black on both ends. YATZEE! I cant tell you how many times i have pulled that relay cover and never saw it. There hiding behind a crusty rotted piece of wire loom was the answer.

After following all the instructions, setting timing and clearing codes i was ready for the test ride. First crank i could tell it was right, no question. Took of down the driveway and backed out. Out of habit i feathered the throttle but there was no need. Nice and smooth all the way down the block. I cannot believe the power i was missing!

Once again hats of to Chevy Tech and all the people in this forum for the info they share.

Re: Another Fuel Pressure Question
 

You’re welcome!

Glad to hear things worked out well.