Misfire when accelerating - at wits end.

[Tinker1980]

I have a 1989 Chevrolet C1500, with the 5.7 liter throttle body injected engine, a 5 speed transmission, and 180,000 miles. It starts and idles like a new truck, no misfire at all. When running a constant speed, flat ground, on the highway, same deal - runs great.

However,

Any time a load is placed on the engine, such as accelerating, or when going up a hill, the engine will stumble and miss. Sometimes it's all it will do to get up a hill in the neighborhood where I live. It's also hard to pass people, sounds like one or more spark plugs isn't firing.

I have replaced, in an attempt to fix this problem:

Distributor cap and Rotor
Spark plug wires
Spark plugs
fuel filter

I have tried routing the spark plug wires several different ways to keep them from metal parts of the engine. The check engine light does not stay on, but it comes on when I start the truck so I know it has a bulb. I've tried removing the heat shields that hug the plug boots on the exhaust manifold, didn't help so I put them back. It won't miss at all when in neutral when the engine is revved, only when there is a load. It's done this for 2 months now, nothing I try makes it the least bit better or worse. I suspected the fuel pump, but doubt it would have been "kinda" working for 2 months, it would have stopped or at least gotten worse.

I love my truck, and miss it's power and smooth sound - can anybody give me any suggestions?

-Mark

[Boog]

The very first thing you need to do is have the fuel pressure checked. You have to tee into the fuel line down on the frame at the filter. On the tbi engine it should have 11 to 13 psi if I remember right. If this checks out I'd look at the egr valve and egr solenoid. A bad egr valve can open too soon causing a miss, stumble. Disconnect and plug the vacumn line going to the egr valve and drive it to compare.

[ChevyTech]

Here are a few more ideas:

Don’t overlook the possibility that the ignition coil has low output.

On these distributors the housing/upper bushing area gets 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 a magnetic material. Any of the distributors can do this if they get very worn.

A bad TPS (Throttle Position Sensor)

It would be a good idea to check for computer trouble codes. Let us know if you need instructions on doing this.

[Tinker1980]

When you put a "T" in the fuel line, what fitting needs to be on the "T" to put a fuel pressure gauge on it? I can watch the injectors when the truck is running, it makes a nice little even fan on both sides, gets a lot thicker when you gun the engine. (neato) I would assume the volume of fuel sprayed into the engine is much greater when there is a load?

How would you test the throttle position sensor? And if needed, how big a deal is it to change the ignition coil?

-Mark

[ChevyTech]

When you put a "T" in the fuel line, what fitting needs to be on the "T" to put a fuel pressure gauge on it? I can watch the injectors when the truck is running, it makes a nice little even fan on both sides, gets a lot thicker when you gun the engine. (neato) I would assume the volume of fuel sprayed into the engine is much greater when there is a load?

Here is some info about fuel pressure testing I saved from a previous reply I made to a similar post. Hope the links still work.

C/K truck TBI systems do not have a fuel pressure test port.

The fuel pressure is tested by using fittings to “T” in a pressure gauge. I like to T in the pressure gauge where the fuel filter is located. If the pressure is good, both the pump and regulator are ok. If the pressure is low it could be the pump or the regulator. If the test is done with a method that allows the fuel filter to be in place, a dirty filter can also cause low pressure. The regulator is inside the TBI unit. Stopping the flow in the return line momentarily will test to see if a low pressure problem is caused by the regulator. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure. If pinching off the return line, using care not to damage the return line hose, makes a low pressure reading surge above the specification pressure, then the regulator is the problem.

The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.


Here are some web sites showing fuel pressure test equipment. The first tool gets installed where the fuel filter goes. Most people use the tool in the third site next to the TBI unit.


I you already have a pressure gauge like shown in the second web site below, all you need is the tool shown on the first web site. If not, the third would get the pressure tested for the least money.

http://www.sjdiscounttools.com/ta37650.html

http://www.actron.com/product_detail.php?pid=16174

http://www.actron.com/product_detail.php?pid=16175

http://www.cfm-tech.com/catalog/fuel...er_2940807.htm

Another addition to the list:
http://www.tradervar.com/cgi-bin/sto...;page=3641.htm

How would you test the throttle position sensor?

As for TPS throttle position sensor testing:
Piercing the wires to do testing can cause future problems.
I unplug the sensor, and use jumper wires between the sensor and connector, when I do testing, which is easy when you have wire terminals like the factory uses. (Save old parts to take the terminals out of them)
Test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
It should be about .5 volt at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle.

Spec .5 to 1.25 volts with throttle closed.

Many people just test it with the engine off using an ohm meter on the terminals of an unplugged sensor to test for a smooth change in resistance as the throttle is moved.

And if needed, how big a deal is it to change the ignition coil?

Two bolts hold it to the intake behind the TBI unit.

[Tinker1980]

Checked the coil with an ohmmeter - all the checks were OK, could the coil still have a weak output?

-Mark

[Tinker1980]

Tested the TPS - I tried testing between all 3 terminals and ground, and tested the terminals against each other, with the ohmmeter. Every time I got an infinite reading. Should I assume it's bad?

-Mark

[ChevyTech]

An ignition coil can have the correct resistance and still have low output.

It sounds like the TPS is bad.

When reading some ford information I remember reading that some older analog ohm meters have currant flow that is so high it can damage the TPS used on early 1980s ford systems.

If you are using a digital meter and can get an infinite reading it sounds like you may have found the problem.

[Tinker1980]

Nope, wasn't the TPS.

removed distributor cap to check the pickup coil, which by the way the connector is a real pain to disconnect, and couldn't test it due to worrying about breaking the connector, and I found a small piece of metal stuck between the rotor and whatever that is that's below it. I pulled the rotor off and got the piece of metal out, and tried to drive the truck... it ran GREAT! just like it was supposed to! For about 1 mile. Then, surprise surprise, it started to do the exact same thing it was doing before. Pulled the distributor cap again, no metal of course, made sure the rotor was on correctly, nothing wrong but still have the problem, except for a short time.

I did notice that the central electrode in the distributor cap, that rides on the rotor, had a weak spring. or it seemed like it. Didn't take hardly any pressure to push it back in. I'll go check a new distributor to see if it is different.

[TennesseeZ]

I had the same problems when I bought my 89 K1500. Wound up replacing the distributor, plugs, put 8mm wires and a 48000V? coil on it, and solved that problem. Like ChevyTech says, those distributors wear out. Mine didn't feel like it was worn out, it wasn't sloppy in the shaft or anything, but the new one seems to have been the trick. Try a newer one if you have a spare?

[Tinker1980]

I feel kinda silly...

Turns out there were 3 things causing me problems.

1.) a small bit of sheet metal stuck between the rotor and pickup coil, keeping the rotor from going all the way on the shaft

2.) The brand new set of plug wires I bought a month ago, were no good.

3.) When I changed the plug wires, the porcelain part of the #5 spark plug came off in the plug wire boot.

The engine doesn't miss a beat now.

Thank you all for your help, I learned a lot about testing parts, and what to check!

-Mark

[ChevyTech]

Thank you for letting us know what fixed it.