96 burb 7.4 misfire after 5.0 injectors

[mehcs]

Hello lukeapre,

@mehcs did us all a favor by documenting how he upgraded to the 5 0 injectors &
also replaced the original FPR with a new one...and then suffered a long troubleshooting
session afterwards. Eventually, after checking the Fuel Pressure and relating what he saw,
he finally replaced the *NEW* FPR with a 2nd *new* FPR, and it fixed his issue:



EDIT -- Hint: A percentage of the time NEW simply = Never, Ever, Worked.
It's not fair, but true. Just ask mehcs. :-( For a troubleshooter, a new part is not a
constant, but a variable. A lot of times a known-good used part is more trustworthy
than a new part made outside of the old QA system we were all raised to believe in.
And this would explain why experienced mechanics will never throw any original/replaced
parts away until after the vehicle is running well post repair.

The moral of the story? You may not be aware, but in order to prevent a lack of diagnostic
tools from also preventing the sale of parts, you can go to most parts stores and 'rent'
a fuel pressure gauge for $0. (ie: You pay for the rental tool up front, but when you return
it you get a 100% refund.)

Given this, I strongly recommend that you locally obtain a 'free rental' fuel pressure test gauge
from your local auto parts store. Following the test procedure in the Factory Service Manual for
your year GMT400 and report those results here. IF your test results are way off like mehcs's
was, then now you have a baseline to see if the 2nd next part is better, the same, or worse.

Don't skip this step and lose access to the data, for this can lead to some expensive parts
cannon activity.



Disclaimer: I'm an old carb guy coming up to speed on FI troubleshooting, but sometimes you
can experience a 'less than perfect' fuel delivery due to the pressures being out of spec, so
Open Loop (that is, before the engine warms up) the mixture is off, but when the O2 sensors
and the coolant temps warm up, going Closed Loop the system will have compensated for
the inaccuracy via the LTFTs. (Long Term Fuel Trims) That is, the problem will be offset by
the computer until it hits the limits allowed by the engineering team.

Conversely, part of the live data that mehcs shared pointed to the fact that his engine was running
extremely lean due to the bad FPR. Q: How do we know this? Check out the LTFTs he
shared with us during the troubleshooting effort:

You must be registered for see images attach

Essentially, in response to the 'too lean' feedback, the computer kept commanding more fuel
in order to compensate, but it finally hit the +25% limit set in the code. Note that good fuel
injectors that are metering accurately, can only do so when they are fed with clean fuel at the
specified pressure on the fuel rail. Good injectors x too low pressure on the fuel rail = too lean.

And the takeaway from the above is whenever we see the factory computer sitting at +25% LTFT
that the issue is so bad that it can't tune around it. (!)

****

I don't mean to come across as the guy who states the obvious, but even though you "are not a mechanic"
as the owner of this issue I'd like to see you take advantage of a 'free' fuel pressure gauge, follow the test
procedure, write down the results, and share them with us.

And I'd also ask you at this point to also use enough of a scan tool that will allow you to at least capture
the long & short term fuel trims. Bonus points if the scan tool can also accurately monitor any cylinder misfires.
The most affordable options are to use a smart phone & a OBD2 wireless device, while others choose to use
a pro-level dedicated scan tool that is more spendy, but now you can do bidirectional stuff like performing a
balance test between injectors, look for issues that have yet to flag the SES light, but are being stored in
the Mode $6 area awaiting a reporting threshold crossing, etc.

For what it's worth, a lot of times we are troubleshooting vehicles that run fine cold, but go wonky once
the O2 sensors warm up and the system goes 'Closed Loop'. Therefore I find your engine bay interesting
because since the replacement of the injectors & the FPR it isn't quite right until the engine goes 'Closed Loop'.

I'm thinking that if we fix your problem by collecting the data first before fixing, we could end up with some
solid information to add to this thread. Good stuff.

Best of luck --

Well said! Hope you figure it out!

 

[Road Trip]

Now I’m a bit more motivated to actually figure out what my issue is. I start it, runs fine for about 5 seconds, then starts to surge and rough idle. It never dies. Soon as my dash temp needle moves to read 100 it smooths out and runs fine. I kept my old fpr. I appreciate the feedback. Honestly I was going to ride this out until it blows up and get a new engine hahaha, but the truck keeps on going. I’m at 190k miles now with no signs of it dying other than the rough idle at start. When I get going with this I’ll start a new thread. Thanks.

It's a strange thing, but I for one can't force myself to just read the FSM in order to learn
for the sake of learning. On the other hand, when I'm trying to troubleshoot a tough
problem, I can effortlessly scour the manuals and wiring diagrams for as long as it takes trying
to prove/disprove different fix-it theories?

And what you may learn on the old 190K motor in your GMT400 may prove to be very helpful
with other vehicles in your fleet, no matter if they are older, newer, or even made by another company.

When the time is right, I look forward to your new thread, and given the talent in this forum, if
we can collect the data right when the engine goes from good > bad > good again, the answer
should stand out against the background noise.

Safe travels --