Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
No Codes but acts like it shuts off then back on at steady throttle
2000 old style ext. cab gmc salt 4x4 w/454 or 7.4l with 5 speed manual & manual transfer case shifts. Latest problem is sometimes when I start it, it idles erratic. Meaning rpm jumps from 750 to 450, then 850 to 250, then 1000 to 100 rpm then stops running. Sometimes it just idles normal. 750 rpm and smooth. Sometimes on the interstate or local roads at steady throttle it jumps, like turning the key off & back on for a second. Very pronounced at low rpm and more often at slower speed. Get around 2000 rpms or above and it’s not as bad. I do have stock replacement injectors that flow just a little more than stock. I’ll look up the type & have that for you if needed.
I don’t have a scan tool like snap on modi’s or versus to do troubleshooting. That’s part of why I’m here. I have done fuel checking. Pressure gauge w/pump running only get 59 lbs of pressure. Pump not running & key still on I get 51 to 52 lbs pressure. Key off it held that pressure for 35 min. At idle it only holds 49 lbs pressure,750 to 800 rpm, snap the throttle and it goes to 59 lbs pressure then falls back to 49 lbs pressure. I can’t get to the vacuum line on the fuel pump regulator to disconnect. VERY bad spot! So I changed the fuel filter. All readings the same but one, with key off it will not hold pressure now falls more than 5 lbs in 5 min.
I have checked all connections. No leaks, no pinched lines, no flow restrictions. The lines under the truck by driver door look to be braided lines. Can’t pinch off there.
I unplugged the maf sensor while running and it stopped running. Oh well.
this is just extra info. The lower intake manifold is leaking antifreeze and I will be taking it apart and changing gaskets and bad stuff I find. I don’t think this has anything to do with my fuel pressure problem. It was there before intake started leaking.
l will get with you on the other forum about getting a good snap on diagnosis scanner that will hook to many different vehicles I work on. I’ll have to learn how to use it from you I hope. You seem to be very knowledgeable in this area. I’m not. But do have mechanical skills.
Thank you for your time. Maybe this will be useful in you classes.
Have a good day.
Terrell
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