Why you always check fuel pressure first - 1991 K1500 4.3L V6 TBI crank-no-start

[RustyBucket99]

1991 Chevy K1500 Crank No Start

Engine: 4.3L V6

Trans: 5 speed manual

 

Background:

The truck is owned by my friend’s father. I have been in the business of flipping cars, and this truck has not run for about a year, so he gave me a call. Long story short, it wasn’t a good flip for me, but I got the diag job. This truck has been towed around to a few different shops with no luck. The problem started when the owner replaced some exhaust components and when he took it down off of the lift, it would not start. Since then, it has allegedly had the distributor and starter replaced (starter was burned up from cranking). The front cover was removed to visually inspect the timing chain, and it passed visual inspection (I have had a bad timing chain on my Jeep and all the timing marks still lined up).

 

When I looked at it with buying interest:

My first observation when I cranked it was an uneven cranking, indicating uneven or low compression (a reason why I turned down buying this truck). I also never heard the fuel pump run, but that doesn’t always mean something. There is no tachometer on the dash, so I was unable to look for an RPM signal (the tachometer probe is directly wired to connector C111 of the ignition coil and the tachometer, which I assume would be used to provide datastream with the proper scan tool). The service engine light illuminates with the key ON, which is a good sign – this will make diagnostics easier, since I don’t have to chase the bulb circuit to verify that the bulb is not burnt out (no MIL lamp indicates an ECM issue).

 

My plan for day one:

While my first thought is a compression or timing issue, I will first pull codes from the ALDL connector. Then I will hook up my fuel pressure gauge and check for spark. The owner suspects an ECM issue, but given my first look and the nature of the GM bypass ignition system, I do not think the ECM itself is bad. There are, however, PROM (programmable read-only memory) and CAL-PAK modules on the ECM, that do not come with a replacement ECM. These are supposed to allow fuel delivery to the engine if there is a faulty ECM. With this GM bypass system, the distributor is wired to the ignition coil, which [the ignition coil] then sends spark to the distributor cap. The distributor is also wired to the ECM with a 4-pin connector that contains the high/low (ground) reference, EST wire, and bypass wire. Interestingly enough, the bypass wire has connector C114 on it. I do not know the reason for this connector, but it is a potential source for an open circuit (I may end up doing a voltage drop test, but not yet).

My current plan is to pull the codes, follow chart A-3 (in 1991 GM Fuel and Emissions Repair Manual), and possibly pursue any trouble codes. A vacuum test may not help me, but it is easy and I have the gauge so I will just go ahead and hook it up to look for anything irregular (I just pulled the fuel pump relay on my 1993 Jeep YJ and measured cranking vacuum, it bounced from 0-5 psi, so I have that as a guide). I will probably end up doing a compression test as well, and then a leak-down test if the compression test guides me in that direction.

 

Day one findings, and plan for day two:

I checked spark at the coil and put a fuel pressure gauge on the system. There is spark from the coil and 6 psi of fuel pressure. I should be seeing 9-13 psi, and the system doesn’t hold pressure, but I would assume that the engine would run (very poorly) on 6 psi. The injectors are spraying into the throttle body. I didn’t like this low fuel pressure, but I moved onto other things because the plugs are fouled, they obviously aren’t igniting the fuel. Vacuum gauge bounced from 0-3 psi. Compression was decent, 90 psi on 3, 100 psi on 5, 130 psi on the other cylinders. I did a leakdown test on #3 and I could hear the pressure leaking into the crank case. Probably some worn piston rings. This is just barely out of GM’s specs from the service manual, but it confirms the low compression I heard while cranking.

I pulled codes 33 (MAP voltage high), 42 (EST circuit), and 54 (fuel pump low voltage). I checked B2 from the ECM to ensure that the test light illuminates. It does. There could be some resistance on the circuit somewhere downstream from where I tested, but the test light wouldn’t be that bright if there weren’t at least 11 volts. I cleared the codes and none have come back.

Chart A-3 in the service manual states that a no-start condition with spark that shows fuel spray from the injectors indicates a faulty injector or faulty injector seal. This makes no sense to me, perhaps it is a writer error. Still, I want to look at the fuel injectors a little more closely. I will have to check the resistance on the injectors, and then unplug them and see if they spray with the fuel pump running.

My first plan for day two is to check the engine cylinder head ground for a voltage drop. If there is a bad ground, I will get a no spark condition at the plugs. Next I want to look at fuel pressure again. I will move the fuel pressure gauge from the filter outlet to the filter inlet, looking for fuel restrictions in the filter. If all is good, chart A-6 in the manual states to pinch off the hose on the gauge to deadhead the pump. My gauge is not set up this way, but there is some soft hose on the fuel line. Maybe it is soft enough to pinch closed. If I still do not get 9-13 psi, I will drop the tank and check power and pressure directly at the pump.

Other ideas: O2 sensor short (because they were replaced prior to this no start), PCM interference with coil while cranking (I will try to unplug it from the coil, I think the coil’s connectors are one piece together – the coil should fire based on the pickup coil in the distributor while cranking), checking for oil/coolant in fuel, then ignition timing (I may try to get better with a scope – I can connect a pressure transducer into the cylinder and verify that the dropoff of the voltage pulse into the ignition coil lines up with peak pressure in the cylinder).

 

Wrap-up:

The owner called me. He played with the distributor and found that it is 180° out. He fixed it, and now the truck stumbles but still won’t run unless gas is poured down the throttle body. NOW… the final call is a fuel pump and fuel filter. It is confirmed that the original source of this no-start was a faulty fuel pump. And with all the hands that touched it, more and more problems were created. A shop put in a new distributor which then guaranteed that this truck would never start.

 

The VERY FIRST thing I did was hook up my fuel pressure gauge, then test spark. If the shop would have checked fuel pressure and saw 6psi, they would have replaced the pump. You need fuel, air in/out, spark at the proper time, and compression. If you have those four things, your engine will run. Their “fuel test” was probably to pop off the air cleaner and see if the injectors were spraying fuel.

 

Tools used:

OTC 4480 fuel pressure tester with Actron 0180-000-1465 GM TBI adapter

Incandescent test light

Vacuum gauge

Compression test kit with homemade leak-down tester

I don’t have a Tech 1 scan tool, I want one, but I probably don’t need one

[Brian Lindsey]

if you can't hear the fuel pump run , it usually means its not running , fuel pumps are not quiet, we have to start somewhen on diagnose  crank no start , fuel pressure is easiest , then move to spark , and so on , on my 98 with a 5.7 my fuel pump ran , but only putting out 30 lbs  psi , my truck requires 60 lbs psi , so even though pump ran , it wasn't putting out enough pressure to start the truck

Edited April 8, 2021 by Brian Lindsey