I have a question on a 1993 GMC C K Series 4X 7.4L Engine

Tiny
J-WARDLE71
  • MEMBER
  • 1993 GMC C3500
  • 7.4L
  • V8
  • 4WD
  • AUTOMATIC
  • 183,592 MILES
Well to start with the truck had issues with Misfiring and Backfiring through the Carburetor, Replaced the Distributor Cap & Rotor, Ignition Control Module, Spark Plugs & Wires and set the Distributor Timing to specs at 4 degrees BTDC and test drove and the vehicle drove really good for the most part but was still Backfiring through the Carburetor Occassionally at 3/4 Throttle! Do you have any idea what could be causing the problem? Also have a Hard starting concern (Cold or Hot), I noticed the Fuel Injector wires were melted and touching and I replaced the wiring to the Injectors and retested and still a hard start! Fuel won't start spraying through the Injectors until about 30 seconds to One Minute of crank time and the Injectors will start spraying and the truck will start! So I was thinking that it could possibly be a defective Fuel Injector/ECM Module or whatever controls it! Or it could possibly be a Fuel Pump concern. What do u think it is or could be causing the problems I'm having? Any ideas, diagnosing or repairs to fix this problem?
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Wednesday, November 12th, 2014 AT 9:28 AM

7 Replies

Tiny
HMAC300
  • EXPERT
Check for a vacuum leak at intake and under the tbi unit. Also see if you can find the cylinder that is backfiring by puling plug wires one at a time. If you don't get a good ignition signal that is what tells the computer for the tbi unit. DI am assuming you replaced the module in the dist and hopefully used dielectric compound on bottom of module
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Wednesday, November 12th, 2014 AT 9:40 AM
Tiny
J-WARDLE71
  • MEMBER
I checked for a vacuum leak under the TBI Unit and the Intake Manifold and no signs of any leaks. As far as the Backfiring goes I'm assuming u want me to pull each plug one at a time because it's possible that a Intake Valve in one of the cylinders may be sticking? Because as far as the Ignition System goes, I checked the position of the Rotor in the Distributor with # 1 Cylinder at TDC on the compression stroke and the Rotor was pointing to the #1 Plug so that was good plus I set the timing while running to 4 degrees BTDC and it was running good at that point, so with the ignition set properly their is no way the timing could be off unless the Timing Chain is worn and possibly jumped a few teeth or like I said one of the valves is sticking! Also on the Ignition Module I replaced in the distributor I did put dielectric compound on the bottom of the module because it said on the box it came in that if u didn't it could damage the module due to heat build up! I'm actually a Auto Technician myself but only on Ford vehicles have I been trained on and was fully Certified! I went to school at UTI for a couple of years also, but The older Chevy/GMC Vehicles I'm not to familiar with so I thought I would ask someone who has had experience on this particular vehicle! The concept of things are the same with any vehicle but their are some differences on the older vehicles to the newer vehicles plus each Manufacturers vehicles can be set up differently and without Wiring Diagrams and other Diagnostics to help u pinpoint the problem makes things difficult.
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Friday, November 14th, 2014 AT 11:32 AM
Tiny
HMAC300
  • EXPERT
Fords chevys they all work the same lol. IT could be a dist cap that makes it backfire as well so check for cracks and carbon tracing in cap which will be shown by black lines in cap. It is probably not the intake sticking but a cam worn out and the reason I said to try to see what cylinder it was so you only need to pull one valve cover and look for a worn cam lobe. With a worn lobe the valve won't open far enough if at all or a bad pushrod/rocker arm as chevys sometimes push the push rod through the rocker. Hey I worked on fords for many years as well.
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Friday, November 14th, 2014 AT 12:00 PM
Tiny
J-WARDLE71
  • MEMBER
True they do work the same Lol! Just might have a different layout is all! Yeah with the new Distributor Cap I didn't see any tracing or anything unusual. Oh yeah your right, a worn Camshaft lobe could cause that same problem! So Chevys are natorius for their push rods being pushed through the Rocker Arms huh? I guess if I can find which cylinder is misfiring I can do a little investigating and narrow it down to the cause of the problem! As many miles that are on this Engine, I wouldn't be a bit surprised that it's a Mechanical concern of some sort for the Backfiring. But as for the Fuel Injectors not spraying fuel, what could be the cause of that problem if their isn't a vacuum leak? I was thinking it could only be 3 things that I could think of, either a defective ECM, defective fuel injectors or fuel injector assembly or the Fuel Pump could be going bad. Could the Sensor that Monitors the Vacuum to the TBI also cause that issue if it's defective? What is that Sensor called?
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Friday, November 14th, 2014 AT 1:17 PM
Tiny
HMAC300
  • EXPERT
The injectors need a signal form the dist to fire so it may not be getting that. That is OBD1 and along time since worked on one of those. Lol
these older models have what is called a prom inside that is removable it could be that or a bad ecm for the injectors. You really need a tech 1 to check that out which is a gm tool or something that will read OBD1 like a pro model not the other styles. Although you can turn key on and cross A & B on the aldl and watch service engine light see if any codes come up. They go in series of three like 1 then 2 is code s 12 etc. That may show something you only need a paper clip for that. The aldl is on bottom of dash by steering column
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Friday, November 14th, 2014 AT 1:53 PM
Tiny
J-WARDLE71
  • MEMBER
Yeah OBD 1 is out of my ball park also! Lol! I got to use OBD 1 Scan Tools when I went to UTI but haven't touched one since! I've only been Auto Teching for 6 years and Everything I have dealt with is OBD 2. OBD 1 is pretty much almost obsolete anymore! The dealership I used to work for threw most of their OBD 1 tools and diagnostics in the back room because most of the stuff we dealt with was OBD 2, and I have a code reader of my own for OBD 2 but it's no use for me on this truck I'm working on! The company I'm working for now just has me repairing their vehicles on the job site and most of them are Chevy vehicles some are older and some are newer! They do have a few Dodge Cummins trucks also! But without the proper diagnostics, it's pretty much a guessing game! But with what knowledge I have and the help I'm getting from u, maybe I can get this vehicle figured out! I've almost been Tempted to replace the ECM because won't it come with a new Prom also? At this point given my situation would u just say replace the ECM and take a chance? These trucks this company uses is off road kind of like a Construction Company and the area they drive in is bad! These trucks get the hell beat out of them so by the looks of it, these vehicles have been worked on a lot!
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Friday, November 14th, 2014 AT 2:20 PM
Tiny
HMAC300
  • EXPERT
Those ecms do not come with a prom and to get one is almost impossible but check it out and see what you can find first. If you had a manual to help would be much better also check coolant sensor for resistance and see if map sensor vacuumline is plugged or rotted/soft that creates problems as well. But I you get codes is much better help.
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Friday, November 14th, 2014 AT 3:05 PM

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