Major Misfiring in a 1991 5L V8

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

JustinC

Newbie
Joined
Jun 16, 2022
Messages
1
Reaction score
0
Location
Ontario
Link to video of issue In the video I was full throttle and there was little engine response.

Truck has 2 codes in from obd1. Code 32 EGR Loop error, and Code 44 O2 sensor lean exhaust gas.

Trucks tachometer started bouncing like that about a month and a half ago, asked some people about it and thought it was just the tach since engine was running by 100%.

Just yesterday it misfired once than started working completely normal rest of way home, got home checked all connections I could find none seamed loose. Today started bogging down bad, but as soon as you shut it off and turn it on it INSTANTLY starts working fine and can rev like normal, what sensor should I look into swapping first or could this be the main ecu starting to go?
 

Frank Enstein

Best. Day. EVER!
Joined
Jul 15, 2018
Messages
2,005
Reaction score
3,431
Location
Canton, Ohio
A misfire on a cylinder can lead to a false lean from the O2 sensor.

If a cylinder misfires then all the oxygen it was supposed to burn gets dumped into the exhaust.

But absolutely change the fuel filter first!

Then check/clean all the ground wires.


If the tach went wonky it may be an ignition module beginning to fail.
 

DerekTheGreat

Forum Regular
Joined
May 23, 2016
Messages
1,557
Reaction score
1,557
Location
Michigan
Based on some discussions I've had with members and like what Frank has said, I suspect your ICM is going bad. I believe a lean condition is what PlayingWithTBI said he experienced when he saw tach flutter. Mine does that occasionally too, just hasn't given me a reason to tear it apart just yet.
 

Nad_Yvalhosert

I'm Awesome
Joined
Oct 11, 2016
Messages
1,666
Reaction score
2,085
Location
Rochester, NY
A stuck open (or falsely commanded open) EGR valve will cause every condition you're describing. Inert gases taking place of air and fuel in the cylinder, causing poor running and lean exhaust.
Since you reported code 32 first, I'd diag the EGR before you look at anything else.

All the true misfires I've ever seen have created a rich condition from all the unburnt fuel. And that's usually an ignition related event.
Its the info from the O2 sensor that the ECM uses to lean the mixture as a result of rich exhaust.
And seeing as you have only one O2 sensor, you're only "reading" the driver side half of the engine, but both injectors.

A true lean condition would be either stem from a vacuum leak or low fuel pressure and generally affect the entire engine. It's not due to bad plugs or wires, in fact the engine will run much better if you spray starting fluid into the TBI (as it's already running) as well as using a flammable spray on vacuum hoses, intake and TBI base gaskets. If the engine revs up, you found a leak...

Low fuel pressure is hard to diag on a TBI system, but try clamping off the fuel return hose at the tank. That'll spike the pressure and force it richer. If the engine runs better, think pump.

And filter...
 
Last edited:

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
10,969
Reaction score
13,751
Location
The Seasonally Frozen Wastelands
Try changing your fuel filter.
...and test fuel pressure.

A misfire on a cylinder can lead to a false lean from the O2 sensor.

If a cylinder misfires then all the oxygen it was supposed to burn gets dumped into the exhaust.
True.

If the tach went wonky it may be an ignition module beginning to fail.
Or pickup coil (including the magnet on the distributor shaft), or wire harness between distributor and ECM. In rare cases, it could be the ECM itself.

Based on some discussions I've had with members and like what Frank has said, I suspect your ICM is going bad.
Could be...but could also be the other stuff already mentioned.

I believe a lean condition is what PlayingWithTBI said he experienced when he saw tach flutter. Mine does that occasionally too, just hasn't given me a reason to tear it apart just yet.
Sudden tach movements in the downward direction tend to be a lack of ignition signal. The tach is getting regular signal appropriate to the RPM. Suddenly, it gets ZERO signal, but before the tach can swing to zero, the signal comes back. So the tach drops suddenly, then recovers.

Slower tach movements up or down might be more related to fuel mixture.

A stuck open (or falsely commanded open) EGR valve will cause every condition you're describing. Inert gases taking place of air and fuel in the cylinder, causing poor running and lean exhaust.
Since you reported code 32 first, I'd diag the EGR before you look at anything else.

All the true misfires I've ever seen have created a rich condition from all the unburnt fuel. And that's usually an ignition related event.
Its the info from the O2 sensor that the ECM uses to lean the mixture as a result of rich exhaust.
The engine has an O2 sensor, not an "unburned fuel" (Hydrocarbon) sensor. The sensor doesn't register the unburned fuel, it registers the "unburned" oxygen. As far as the computer knows, excess oxygen is a "lean" indication, so it commands additional fuel.

The exhaust smells "rich", even if it's a lean-misfire.

But diagnosing the EGR is completely reasonable.

And seeing as you have only one O2 sensor, you're only "reading" the driver side half of the engine, but both injectors.


A true lean condition would be either stem from a vacuum leak or low fuel pressure and generally affect the entire engine.
If the O2 sensor is working properly, it'll cause the computer to go "rich command", which then drives the short- and long-term fuel trim numbers high.

Scan tool monitoring can be very helpful.

Low fuel pressure is hard to diag on a TBI system, but try clamping off the fuel return hose at the tank. That'll spike the pressure and force it richer. If the engine runs better, think pump.

And filter...
Only if the pump has the capacity to achieve higher pressure. If the pump has failed to the point where it's already at it's max-pressure, and that pressure is below the regulator pressure, there's no fuel being returned--so clamping the return doesn't change the pressure.

There's really no substitute for installing the pressure-test adapter and using a gauge to assess pressure.
 
Top