View Single Post
Old 10-29-2016, 03:12 AM   #46
98extracab
Registered User
 
Join Date: Oct 2016
Location: California
Posts: 3
Re: 88-98 FAQ's "Is your truck running weird? Check here it could help"

Quote:
Originally Posted by ChevyTech View Post
TBI (Throttle Body Injection) powered trucks, common problems.
In one spot, as short as I could make this post.

It is common for the gasket under the TBI unit to leak vacuum causing idle issues.
The throttle shaft can wear the housing creating a vacuum leak on the driver’s side of the TBI unit.

Testing the fuel pressure is a good idea if the idle is erratic or poor. I wish they would have put fuel pressure test ports on these trucks. The fuel filter, fuel pump, fuel pressure regulator (broken spring) and the rubber that connects the fuel pump to the sending unit are all possible causes of low fuel pressure. Low fuel pressure on these TBI trucks shows up as a poor erratic idle, and slow – hard starting, and just because it runs ok on the the freeway under heavy throttle you should not think it has good fuel pressure.

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. These distributors can also have problems because of loss of magnetism needed for the pickup coil to work, which cause more problems at low RPMs. If the magnetic material is cracked in pieces you should replace the shaft or the entire distributor.

Distributor modules are a common failure part. They can fail as they heat up. They can fail causing a no start, because of no spark. They can cause the injectors to not get triggered by the ECM even if there is spark. Or they can fail causing all kinds of poor running conditions. Many parts stores can test them. Make SURE you use heat sink compound under the module when you install it, or heat made in the module can't get dissipated and will destroy the module. Many replacement modules fail VERY quickly.

A bad TPS can make it so that the ECM does not know the correct throttle position and the ECM will not move the IAC. A TPS that shows wide open throttle can cause a lack of fuel for starting the engine because the system goes into the clear flood mode and cuts the fuel injected.

Coolant sensor failure on the sensor the the computer uses to set the mixture can cause the mixture the engine gets to be wrong. The most common problem it causes is a mixture that is too rich.

EGR valves can get carbon chunks holding them open. They can also drag on one side of the plunger and catch, sticking open at times. The EGR solenoids can also leak vacuum to the EGR valve at idle. If there is ever vacuum getting to the EGR at idle, unplug the electrical connecter to rule out an electrical problem, and if there is still vacuum getting to the EGR valve at idle replace the EGR solenoid. The solenoids can suck dirt in and stick.

The most common EGR system on the light duty TBI trucks is a “Negative Back-pressure” EGR system in which the EGR valve itself modulates the EGR opening. This system is very picky about the EGR valve operation and low quality replacement EGR valves can cause drive-ability problems that people are unable to find the cause of.

A common condition caused bad a bad EGR valve is a hesitation, or flat spot, at light throttle acceleration.

The TBI trucks from 1987 to 1995 are the OBD1 system and the computer trouble codes can be read without a scan tool. All you need is a paper clip.

You must unplug the set timing connector (wire) to set the ignition timing on these trucks, and if you don't the timing will be wrong. This will set a trouble code.

As these trucks age head gaskets leaking between the cylinders is a problem. As the head gasket fails, often the first symptom is pinging, and sometimes it causes a trouble code caused by high knock counts.

The none-roller lifters used in these engine along with the changes in motor oil removing ZDDP has shown up in the number of flat cam lobes on high miles engines.

Trying a different brand of gas is another thing you can try. It has been my experience that the TBI trucks do not idle as well on ethanol fuel and as they raised the ethanol content this was noticed by many people.

With a search you should be able to find discussion on all these topics in old threads.
My friend has a 94 5.7 and at around 1500 rpm or so and deceleration it hesitates/bogs and has a code 43. We had recently changed the motor out because it was knocking and it ran great until about a week after and started having these issues. From what I read on the egr failure it seems more like that. He still has his old egr valve and will be throwing it on since there were no codes when that engine was in. But I was just wanting an input to see if that may be the issue.
98extracab is offline   Reply With Quote