Engine running rough

Hi,

I checked for technical service bulletins and none are indicated for this. Has the check engine light come on when this happens? If so, have you scanned it for trouble codes?

If not, the next thing I would be checking is fuel pressure. Low fuel pressure is more likely to be noticed when the engine is under a load for an extended period of time. If you have't done it, here is a link that explains how it's done.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Also, if you didn't replace the fuel filter, that would be a consideration as well.

Here are the manufacturer's specs for fuel pressure:

1998 Chevy Truck C 1500 Truck 2WD V8-5.7L VIN R
Pressure
Vehicle Powertrain Management Fuel Delivery and Air Induction Fuel Pump Fuel Pressure Specifications Pressure, Vacuum and Temperature Pressure
PRESSURE

55-60 psi
Open Throttle 66 psi

Let me know what you find.

Joe

About 2 years ago the fuel pump completely failed and I replaced it along with the fuel filter. I did consider the fuel pump to be suspect so I checked the fuel pump pressure, with the engine off "key on" I had 61# with it idling 55#. When I rev up the RPM's it stayed around 55#. No big changes, so I was on the fence on this one. Your thoughts on these results? I will replace the filter to eliminate that. I have not tested my fuel regulator, I will have to get the procedures for testing a 1998 C1500 350 Vortec.

Update: I tested the fuel pressure regulator today, with the key off 61#'s engine running 55#'s, then from idle I snapped open the throttle "fFull" and the pressure jumped back to 61-62#'s. So my regulator seems to be working as expected. The tougher challenge will be monitor the fuel pressure when pull a hill "heavy load".

Hi,

It sounds like it's right where it should be. Are you sure there are no engine vacuum leaks? Have you checked for diagnostic trouble codes? And here is something most people don't think of is the catalytic converter. Is there any rattling from the exhaust which could indicate it is loose internally?

The reason I ask about vacuum leaks is because under a load, the vacuum drops. If there is a leak, it can cause an issue and it could affect the fuel pressure regulator as well. It's only a thought at this point. As far as testing the regulator, don't bother at this point. It is reacting exactly like it should.

Let me know.

Joe

I did a vacuum test as well, At an idle it pulls a steady 21 inches. Your thought on this value? When running up the RPM's naturally it drops a bit but stays very steady. I did spray starting fluid /carburetor cleaner in the different areas and monitor the vacuum and RPM but didn't see any changes, In the recent past I would get a P305, P307 and sometimes P300, That's when I did a full tune up. That then eliminated the cylinder 5, 7, misfire, when I am pulling a hill at 50-60 and if I don't let up when the engine starts to miss. I will sometimes get the P300 "random" misfire. I have not noticed any noise at all from the catalytic converter. I would expect the vacuum test to show if it was clogged. How do you test the fuel pressure when pulling a hill "without a very long hose leading into the cabin"?

Hi,

The best way to check is to watch fuel trims to see if it is running lean. Short of running a fuel pump hose into the vehicle, that's the only way I can think of.

Let me know.

Joe

Do you think 20-21 inches is a pretty normal vacuum reading? I felt it was a couple of inches lower than I expected. How would you watch the fuel trims some type of OBD reader?

Dean

Hi Dean,

Yes, you would need a live data scanner. You can get one for not much money. I don't know if you have access to one.

Let me know.

Joe

The OBD I have only reads codes and clears the non-volatile memory.

Dean

That vacuum level sounds fine. 18-22 is normal for a new engine at idle. I would get a scan tool that can read live data and watch the fuel trims but more importantly the misfire counters. The P0300 now and then could be from a coil breaking down under load and heat.

Do you have any recommendations on one that would be cost effective "home use" that would also run the ABS motor for purging out the air from the module when bleeding the brakes. I kind of like the idea of the ones that it use you cell phone to control it.

Thanks!
Dean

Most of the cell phone units don't allow much if any bi-directional control (like brake bleed or EGR control or injector controls) unless you spend a lot more for the professional versions that cover one make per purchase.

For a DIY set-up it depends on what you really want to do and what you are willing to spend, as well as how many vehicles will you work on? For instance, do you help a neighbor or other family members with their vehicles? Do you have 2-3 different years that you need coverage for? Do you want new or used? How much bi-directional control do you want? There are a lot of newer tools on the market that give you much more than the older tools ever did. As for a particular tool, think about those questions first. However to give you a few ideas on full power but reasonable scan tools search for Autel MS906, Launch Diagun IV. Be careful though, there are some out there that require a subscription for them to work and they have to be online to verify that you have paid!

Did some researching the Auel models I came across the Autel MaxiCheck Pro Scan Tool for around $200.00 seems like it might be good for a DIY person, seems it has some of the features I would need/use, give me your thoughts on this unit, it seems to have good reviews.

Thanks!
Dean

Well it claims to have the ability to read the correct data and some controls. Maybe look on YouTube to see if there are any videos and see what you think. It's had to tell what the real coverage is without having the tool in your hands.