1998 Chevrolet Other Idles rough and then stalls

Tiny
GARY276
  • MEMBER
  • 1998 CHEVROLET
  • 7.4L
  • V8
  • RWD
  • AUTOMATIC
  • 15,000 MILES
1998 Motorhome on a Chevrolet P32 chassis, 7.4l 454 vin code B, 15,000 miles. At highway speed runs fine, plenty of power, no problems. Stop at a red light rpms fluctuate 300-400 rpms and will stall if I do nothing. Put it in neutral give it a little gas and it runs fine, no rpm fluctuations, light turns green, let off the gas put it in drive and it runs fine. First start up in the morning no problems, let it idle for 10 mins and the rpms start to fluctuate and will stall unless you give it a little gas, always starts right up after it stalls like nothing was wrong.

Since this started I have changed the in line fuel filter, plugs, plug wires, dist cap, dist rotor, PCV valve, EGR valve, IAC valve, gas cap, MAF sensor. Removed and cleaned the throttle body.

I checked and had a technician check the vacuum lines, engine and chassis grounds, fuel pump pressure was 50 psi, tech hooked up a scan tool, no codes and parameters all looked normal to him.

Put Lucas tune up in a bottle (x3), 75 gallon tank, and a full tank of premium gas and drove it 400 miles, still doing it.

Of note, the tech pulled the plug off the engine coolant temp sensor and the idle smoothed out, not perfect and took longer to stall, he also sprayed either carb cleaner or something similar into the air intake box with the engine running and the idle would momentarily smooth out.

I understand that pulling the plug on the coolant temp sensor makes the computer give more fuel just like me manually giving it a little gas thus making it idle better.


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Thursday, July 17th, 2014 AT 2:25 PM

10 Replies

Tiny
HMAC300
  • EXPERT
Actually pulling a plug on any sensor puts the engine into open loop and your gas mileage goes right to hell. Try recleaning the throttle plate on both sides with choke cleaner and iac hole. Then check to see if your intake bolts are tight. On these larger motors they had a tendency to loosen and cause a vacuum leak. Torque upper manifold to 6ft lbs then 13 ft lbs lower to 30 ftlbs. You might also spray the choke cleaner around there while running to see if you have any vacuum leaks. Also fuel pressure should be 56-62psi so if you are at 50 it's to low so it may need a pump. But try a filterfirst
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Thursday, July 17th, 2014 AT 4:40 PM
Tiny
GARY276
  • MEMBER
Checked the torque on the manifold, all tight, sprayed choke cleaner on all vacuum hoses and connections and around where the intake manifold mounts, rpms stayed the same during all of this, also changed the coil on advice from a friend, took it for a 6 mile drive and it still does the same thing. Also replaced the coolant temp sensor that the tech unplugged.

If this helps: Today when I first started it up it idled for about 20 minutes and didn't stall and rpms didn't flux, took it for the 6 mile drive and when I stopped and held the brake at a stop sign after a few seconds the rpms start to flux and if I let it go it will stall, starts right back up, I also noticed when I came back and parked it the rpms were steady with the a/c off but when I turned it on the rpms would start to flux, same with putting it in drive and holding the brake, in park the rpms are pretty steady.
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Saturday, July 19th, 2014 AT 9:43 AM
Tiny
HMAC300
  • EXPERT
I see nothing about fuel pressure nor cleaning the throttle plantlike my reply. Also in my reply I stated what fuel pressure should be and yours isn't up to snuff it is not high enough and that may be the problem.
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Saturday, July 19th, 2014 AT 11:35 AM
Tiny
GARY276
  • MEMBER
Re-cleaned the throttle body, looked pretty good since I cleaned it about a week ago, made sure the area where the IAC plunger seats was good and clean and smooth, hooked up a fuel pressure gauge and with the key on pressure is 52 psi and holds, start the engine and the pressure stays at about 52 psi, give it short throttle burst and the pressure goes to about 57-58 psi, then back to 52, when the idle is fluctuating the fuel pressure stays steady at about 52 psi.
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Saturday, July 19th, 2014 AT 3:25 PM
Tiny
GARY276
  • MEMBER
Also my mistake on the vin code, it is a J vs B as in my first post.
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Saturday, July 19th, 2014 AT 3:26 PM
Tiny
HMAC300
  • EXPERT
Fuel pressure hs to be int he range of 56-62 at all times even at idle. Whn you punch the throttle on this engine it should go to 62 psi not 57-58. Either your filter is partially plugged and being a motor home it wouldn't surprise me because there tanks are for crap. Or the pump ison the way out
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Saturday, July 19th, 2014 AT 4:14 PM
Tiny
GARY276
  • MEMBER
Changed the inline filter along with the plugs, etc last week, it was a little hard to blow through, probably the original filter.
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Saturday, July 19th, 2014 AT 4:37 PM
Tiny
HMAC300
  • EXPERT
I have no further comments you don't seem to be listening to what I'm saying.
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Saturday, July 19th, 2014 AT 4:39 PM
Tiny
GARY276
  • MEMBER
Not listening? I have done everything you asked. You mentioned a partially plugged fuel filter as a possible cause, it was changed already, your only advice is "maybe a fuel pump". I guess I exceeded my donation. Now I know why I avoided these type websites in the past, you guys are a rip off.
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Sunday, July 20th, 2014 AT 6:06 AM
Tiny
HMAC300
  • EXPERT
Ya know by your own admission your fuel pressure is not at spec which causes no starts, rough idle and poor power. I don't know what to tell you. If you aren't satisfied recontact me and i'll see your donation is refunded. We can't fix this for you
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Sunday, July 20th, 2014 AT 8:05 AM

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