LOW FLOW pump
This vehicle displayed an intermittent No Start condition. Pretty simple problem. I used propane to start the vehicle when it would not start randomly in the parking lot. Fuel delivery for sure. It changes a little bit here as I took a fuel pressure reading and it displayed 55PSI which is in spec. I suspected a FLOW issue with the in-tank pump at this point as no codes were set. I was gonna do a FLOW test and measure the pump's liquid output but I decided instead to take the Bernie Thompson approach. This is a normal DC 8 commutator pump controlled by a PWM module on a returnless system. I bypassed the module using the GM terminal tools and used an AMP clamp to monitor the commutator pull. Here are the captures. The pulsation of the bad pump could be seen on the scope, pressure gauge, and pressure gauge line. Using frequency we can calculate the pump's speed and therefore as a by-product output. This pump was only rotating at 1368 RPM at MAXIMUM output. Far too low. I removed and replaced the pump and performed the same test. The new pump's rotational speed was 4560 RPM. A massive difference. I found this very interesting and (lack of professionalism coming up) I wanted to show it as proof positive Bernie Thompson is a f**king real life wizard.
You are right he is a wizard and a thoughtful wizard to share his knowledge. Gread work!
Keep this handy as a reference for an 8 commutator pump. Simple method to calculate RPM
Bernie is The Man. Seeing you use Bernie's techniques is fantastic!
Good job figuring out the problem. This testing procedure is nothing new. Techs on iATN were doing tests like this over 20 years ago.
It came out of the old Compuserve forum in the early 1990's(?). People like Jim Linder and Chris Chesney were among the first to teach it and I first saw it when Chris presented it around 1995 or so. I can't say who figured it out first but it's been around a long time.
You are in for a real treat when you hear about ringing injectors and how to fix 'em! :)
You would bring that up! Do you still use a diode across the terminals? lol
My pocket protector is full of diodes to fix ringing injectors. 😂
I’m pretty sure that it was capacitors being promoted at the time! 😅
Too bad, you missed some great debates in the mid '90s-early 2000s. I joined in ‘98 and read about it in one of Jim Linder’s articles, maybe “Fuelish Thoughts” or another on the topic. Regardless of who was the originator of the method, revisiting sound diagnostic concepts clearly still has great value.
Hi Martin, Bubbas Garage and the “Fool Pressure” test. Good times!
Ah, that was the one Scott! Thanks. I had all of those printed off, sitting in a filing cabinet until just a few weeks ago when I started thinning out stuff. I recall one measurement …..“if it shoots over the fender…..!” VBG I just took a look and do still have my saved copy of Tom Clozza's Vacuum Waveform Analysis I miss a lot of those interactions and times we had with some great people…
William, I was blown away by the concept of planetary gear-sets years ago when I went to automatic transmission training. Later I found out that the Model T had them. Later still I learned that the Greeks conceptualized it 2000 years earlier. My point being that PERHAPS there is little that is TRULY new. Sometimes it is just repurposed, refined, reinvented, or even rediscovered like electric…
Sigh, Compuserve on dial up. Those were the good old days!
Good ole days were before web crawlers and if you know that term, you understand.
Yup, I learned that at Jim Linder's guru school way back when. Or should I say “Bubba's Garage”.
Love to see this stuff. What bugs me is even the new commutators are not perfect when I retest new pumps…
Correct Elias, the humps often look terrible, because the components are not bedded in. I've seen techs call brand new pumps bad, from only looking at the humps signature without doing the RPM calculations.
Correct Martin. Another tip is you don’t need to do the RPM calculation, just compare the frequency to known good.
They won't look “good" until the brushes and commutator break in.
I totally agree. I attended an in-cylinder pressure transducer class a few years ago here in OKC that Bernie was teaching. He must be the smartest automotive guy I have ever met. Really easy to talk to, and could explain it in a way anyone should be able to understand. Great class and really convinced me to get the scope out more often and work on my skills.
Certainly a scope has its place. I am lazy. Your work here is impressive, never the less. I watch the pressure when on the road at WOT. If the pressure stays in spec, it is not the pump supply, or the tubing, filter, vent restriction, relief valve or wiring. You proved it from the other direction by use of propane when it would not even start.
FYI, here's a link to a preview of Jim Linder's current ramping course. aviondemand.com/shop/service-c…
You are correct I did. It has been fixed. Good to know I am still human.
"This pump was only rotating at 1368 RPM at MAXIMUM output. Far too low." Are there some normal and minimum specs for fuel pump RPM, that I have never heard of?