92 chevy truck missing at constant speeds

[chevybill]

Hey guys, I am from the 67-72 part of the board. I am the one owner of a 92 c 1500 2 wd, 5spd manual with 280,000miles. I have replaced the distributor cap, rotor cap, coil, plugs, plug wires and under the rail fuel filter.
Regardless of speed, engine rpms, or gear selected when you attempt to maintain a constant speed the truck starts jerking like it is missing or fuel is cutting off. Step on the gas the least amount possible and it will level off and accelerate smooth until you try to maintain a constant speed again. I also put a can of sea foam engine cleaner through the intake via the brake booster vacum hose, following the advise of a local dependable mechanic.
I also put about 6 gallons of fresh premium fuel in with about a half can of seafoam in the gas tank.

I have not made any progress with the jerking yet and after investing 100 dollars in parts and the most of a day I need some assistance. Truck has been well maintained and most everything else to consider is original.

Distributor is original as are the fuel injectors themselves. I sprayed penetrating oil around the intake bolts to check for leaks around them and found nothing, engine speed did not change. It idles pretty slow around 600-700 rpms.

Any suggestions? Thanks for your time.

[ChevLoRay]

Check engine light coming on? I know that the OBDI system is more involved to troubleshoot, but you can get it to display codes by flashing the check light.

Sounds like you've done the normal stuff in trying to resolve your issues. Hopefully, someone with knowledge will offer some of it.

[bwood]

Have you ever replaced the fuel pressure regulator spring. I'm not sure if this would cause your issue, but I do now it is a common issue with the throttle body injected trucks. Have you tried to check your fuel pressure, you do need a special fitting to tee into the line for the gauge. Also I wonder if you might have a timing chain or worn dist gear. When your on the gas the slack tightens up and when you let off, the slack in the setup causes problems. These are just some thought. I bet ChevyTech will have some better answers.

[chevybill]

Where is the fuel pressure regulator spring? The distributor gear had crossed my mind as it is original.
As far as a fuel pressure regulator i do not know where it is unless it is in the tank with the pump. As far as timing chain the truck runs smooth the rest of the time.

thanks for the replies

[bwood]

The fuel pressure regulator is mounted on the back of the throttlebody itself. You will have to remove the throttlebody, and it is mounted underneath. I think it has some tamperproof screws or something like that in the cover. The spring is prone to rusting and falling apart. You can get a regulator kits from your autoparts store.


Here is a post from last year about a source for the part:
http://67-72chevytrucks.com/vboard/s....php?p=2570271

[ChevyTech]

Anytime a TBI truck does not run well it is a good idea to test the fuel pressure.

It is common for the fuel pressure regulator spring in the TBI unit to break and cause low fuel pressure.

The fuel pressure regulator spring is in the TBI unit as bwood stated, but all you need to do to get to it is remove the cover and the regulator comes off with the cover. You will need a Torx bit.

If I had no fuel pressure testing equipment, I would take the fuel pressure regulator apart and check to see if the spring is broken, which is fairly common. The cover gasket my tear but often I get by without tearing the gasket.

Note: the service manuals state that the fuel regulator should not be disassembled for safety reasons.
Realize there is a spring inside so if you take the screws out without holding it together, it could hit you in the face.

It sounds like a lean misfire condition so it could be caused by low fuel pressure, but it could just be a vacuum leak. Depending where the vacuum leak is, it may affect one cylinder more then other cylinders, and cause a misfire just like yours. On a TBI engine, a vacuum leak will speed up the idle, if it is a big leak.

On TBI engines it is common for the gasket under the TBI unit to leak vacuum. Vacuum can also leak through components such as the brake booster, bad PCV valve, or charcoal canister purge valve. I use pliers to pinch off vacuum hoses and listen to see if it slows down.

Sometimes a cracked spark plug can also cause it to run like this.


ChevLoRay
There are two threads on this same issue.
Could you combine them or something?

[ChevyTech]

With 280,000miles on the original distributor, it would be a good idea to take the distributor cap off and make sure the shaft is not loose when you try to move it sideways. Also make sure oil is not leaking out the distributor shaft and getting on the pickup coil in the distributor.

[chevybill]

Chevy Tech, thanks for the in depth reply. As I put the new rotor cap on I did not notice slack in the dist. shaft, but I will check that as well.

[chevybill]

Now that I have my new spring, do any of you have photos of replacing this spring? My chiltons is very sketchy about replacing the spring with the usual warning.

If I understand this, I have to remove the TBI unit from the truck and the spring is inside the fuel meter cover assembly?

ON the 4 screws that you must remove to get to the spring do they go in upside down? If so, do I need some blue thread locker (loctite)? to secure them.

Can the spring successfully be installed and the TBI unit reassembled without purchasing a rebuild kit?

[ChevyTech]

Photo
http://chevythunder.com/tbi_regulator.jpg

Illustration
http://www.snflupigus.com/Partspics/image054.jpg

Photo
http://www.cfm-tech.com/InstallationPhotos.htm

Illustration
http://www.goingfaster.com/spo/TBIdiagram.html

Illustration
http://www.snflupigus.com/Partspics/TBI.JPG

If I understand this, I have to remove the TBI unit from the truck and the spring is inside the fuel meter cover assembly?

I would not remove the TBI from the truck. I would just remove the cover, and the regulator will come off with it.

ON the 4 screws that you must remove to get to the spring do they go in upside down?

Yes. But when you remove the cover and flip it over they will be right side up.

Note: the service manuals state that the fuel regulator should not be disassembled for safety reasons.
Realize there is a spring inside so if you take the screws out without holding it together, it could hit you in the face.

If so, do I need some blue thread locker (loctite)? to secure them.

A little Loctite would be a good idea.

Can the spring successfully be installed and the TBI unit reassembled without purchasing a rebuild kit?

Most of the time.

Be careful when you remove the cover that that gasket does not break. Sometimes I use a razor blade to help get it apart without breaking the gasket.

[chevybill]

Thanks, ChevyTech,

You provide some of the best answers I have ever received.

[ChevyTech]

Originally Posted by chevybill

Thanks, ChevyTech,

You provide some of the best answers I have ever received.

You’re welcome.

[Z-71]

It doesn't sound like a fuel issue. It seems more of a timing issue. I would at least check the timing to see where its at. The distributor gears are known to wear faster than the cam gear so that doesn't help. I'm just sayin.........might be worth a look.

[chevybill]

I installed the spring and it still has the "herky jerky" when trying to maintain a constant speed.

Replaced so far, dist and rotor cap, spark plugs and wires, fuel filter and spring in tbi unit.
280,000 miles original distributor and truck runs smooth as long as you are under acceleration. Seems that I notice the sputter or miss now when idling, tach shows about 750 rpms at idle. When in "herky jerky" mode it will almost die then come back every time, such as if you back up and it starts missing and you depress the clutch and it will sputter, almost die and then come back up.

Thanks for all the help so far.

[ChevyTech]

It is always a good idea to check for trouble codes in the ECM when there is a running problem.

Here are some web sites with the basics on retrieving codes without a scan tool or code reader.

Poor drawing of ALDL but ok instruction + codes
http://www.cfm-tech.com/gm_tbi_trouble_codes.htm

OBDI terminal identification
http://robertpowersmotorsports.frees.../ALDL_plug.jpg

OBDI illustration and instructions
http://www.troublecodes.net/GM/

1985 – 1990 trouble codes instructions
http://www.iroczone.com/specs/TroubleCodes.htm


Another thought is that the TPS (Throttle Position Sensor) could be bad, and the ECM is going into the deceleration mode where fuel is drastically cut. The TBI system does not set a TPS code easily so you may want to test the TPS

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)

With the key on and engine off:
Test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
It should be between .5 volt to 1.25 volts 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.

[chevybill]

Is the TPS the electrical looking black square next to the TBI unit toward the passenger side?

[ChevyTech]

Source/link if photo does not show:
http://www.snflupigus.com/Partspics/i-4_B_L.jpg

[Z-71]

The reason i mention timing is because you described exactly a problem i had with my 97. Idled a little rough, but trying to maintain a constant speed led to bad jerking. Accelerate slightly and it ran fine. It was missing under very light loads. Since mine is an auto tranny it would kick it out of overdrive when it started missing. After replacing all that you mentioned, including the upgraded gm injectors, it still missed. After monitoring it with a scan tool, the cam retard was at -13 degrees and the cam gear had substantial wear. It would run fine under a load because the timing advance added just enough positive to stop the missing but as soon as it pulled the timing it would start missing again. Timing was the last thing i checked after replacing and eliminating all other variables. Wish i would have checked it a lot sooner because it was just too easy to check and fix.

[chevybill]

Thanks z- 71, timing is next on the check list.

[chevybill]

Well I am still missing at constant speeds, but yet to check my timing.
Now when I go to start the truck it will crank run for about 5 seconds die as if key is cut off. Try to start again it might repeat 5 second run or turn over and over to the point you think it is not going to start, and after several tries it will finally start. It acts like the old vehicles used to when they needed a set of points really, really bad. I replaced the spring in the injector system however the old spring looked fine.
Another board member pm'd me about a distributor pick -up also called an ignition coil that goes inside the distributor that was causing a same problem in his 87 van.

http://www.napaonline.com/MasterPage...ck-Up+Assembly

[ChevyTech]

If I were working on your truck, the first thing I would do is test the fuel pressure with the engine running.

If you need information on testing the fuel pressure let me know.

If the fuel pressure tests to be good, the next thing that comes to mind is a bad distributor module.

When the engine starts the module is in the base timing mode, but as the engine start the module switches to electronic spark timing, and the timing adjusted according to a signal it receives from the spark (knock sensor) module while in the electronic spark timing mode.

Many stores like NAPA in my area will test the distributor module for free if you bring it in.

The test results from these store are not always accurate, and some times the will say the module is good when it is not, but for a free test that may find the problem it is worth a try.

Make sure when you install or reinstall the distributor module that you use heat sink compound/grease under it. The compound helps dissipate heat from the module. Without it, the module will fail.

The computer needs to receive a distributor reference pulse from the distributor module to trigger the injectors.
It is possible for the module to produce spark without the computer getting a reference pulse. A bad distributor module or wiring between the module and the ECM/PCM can cause this.

… Another board member pm'd me about a distributor pick -up also called an ignition coil that goes inside the distributor that was causing a same problem in his 87 van.

That is called the “pick-up coil”.
If there is oil coming up the distributor shaft and getting on the pickup coil, the oil can damage the pickup coil.

[chevybill]

Please tell me more about checking fuel pressure, as I have no specialized tools to do this.

[ChevyTech]

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, but the pressure must be tested with the engine running.

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



More
http://www.tooltopia.com/index.asp?P...googlebase_18u

http://www.thetoolmaster.com/toolaid...eradapter.aspx

[chevybill]

Just a brief update, before attempting to start if I pour a small amount of gas into the TBI it will start right up and keep running.
So that probably means fuel pressure problem?
Along with the missing at constant speeds.
Is there a fuel pressure regulator outside of the gas tank? Or is it integral with the electronic fuel pump inside the gas tank? Can this fuel pressure problem be repaired with out removing the gas tank and replacing the entire fuel pump assembly that is inside the fuel tank?

[ChevyTech]

before attempting to start if I pour a small amount of gas into the TBI it will start right up and keep running.
So that probably means fuel pressure problem?

When you turn the key on without cranking the engine, do you hear the fuel pump run for a few seconds?

Have you checked the ECM for trouble codes?

Is there a fuel pressure regulator outside of the gas tank? Or is it integral with the electronic fuel pump inside the gas tank?

The fuel pressure regulator is in the TBI unit on the engine.

Originally Posted by bwood

The fuel pressure regulator is mounted on the back of the throttlebody itself. You will have to remove the throttlebody, and it is mounted underneath. I think it has some tamperproof screws or something like that in the cover. The spring is prone to rusting and falling apart. You can get a regulator kits from your autoparts store.


Here is a post from last year about a source for the part:
http://67-72chevytrucks.com/vboard/s....php?p=2570271

Can this fuel pressure problem be repaired with out removing the gas tank and replacing the entire fuel pump assembly that is inside the fuel tank?

It depends on what is causing the low fuel pressure.
It could be a plugged fuel filter.
It could be a bad regulator.
It could be a bad pump.
It could be the hose between the pump and the sending unit leaking inside the gas tank.