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Username Post: Surging idle and heavy hesitation giving gas        (Topic#343218)
Forum Newbie
Posts: 4

Reg: 03-09-17
03-11-17 07:27 AM - Post#2680852    

Hi all, first time posting. I could sure use some help. Just got truck from my dad 1990 2500 only 120k miles. Drove home from Florida 1300 miles no issues, now starting hesitating stalling out and surging on idle. Done several things and still problems. Found water in gas, dumped added dry gas and new fuel filter, new dist cap/rotor, plugs and wires. Replaced water temp sensor, egr, egr sensor, icm and all new vacuum hoses. Stumped slightly better but still issues surging and hesitation. But the kicker when in idle I remove plug off "t" in vacuum hose to map sensor to test vacuum pressure and rpm's rise and run without surging....? Help please any suggestions. Also I have new map sensor just about to change but tired of dumping parts in and no results. Thanks PClout

"7th Year" Silver Supporting Member
Posts: 3908
Loc: Salt Lake City
Reg: 06-20-09
03-11-17 10:03 AM - Post#2680874    
    In response to PClout

Welcome to ChevyTalk.

Before adding another part, either buy or rent a fuel pressure gauge. You need it to be 9-13.

Have you checked your injector spray pattern? Take a bright light and shine into the throat and watch the spray pattern as you open the throtttle. It should be a fine mist cone all across the range. Any drips or dribbling is a fuel pressure problem or dirty injectors.

It would be worth the few dollars and effort to replace the base plate basket. If it's never been off that you're aware of, it won't hurt to verify.

My '95 had a compromised gasket fright between the ports. It was never serviced before I bought it and I' was just barely at 149K when I pulled the engine for replacement. Found the bad gasket when I removed the TB. Prior to pulling the 4.3, it would diesel a couple extra revolutions when I shut it off.


'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke

Valued Contributor
Posts: 3867
Age: 35
Loc: wv
Reg: 08-29-12
03-11-17 03:25 PM - Post#2680922    
    In response to PClout

I was going to comment, but Greg covered everything I was going to say. What he said is exactly right. If you aren't sure how to check fuel pressure, look in the FAQ.


'88 Chevy K3500, aluminum head roller cam 511inĀ³ stroker 10.5:1 compression, 96 NV 4500, 94-98 grille, 305/70-16 (33x12) BF Goodrich KM2s, 91 cluster swap

"15th Year" Gold Supporting Member
Posts: 18770
Loc: Colorado Springs, CO
Reg: 08-08-04
03-11-17 05:55 PM - Post#2680950    
    In response to bowtie44s

Surges are lean conditions. The mixture is artificially lean. Typically this is due to a vacuum leak (like a bad gasket), a bad sensor or incorrect fuel pressure.

Check the ECM for trouble codes.

When you pull the vacuum hose going to the MAP sensor, that introduces a vacuum leak near the MAP sensor, which cause the MAP sensor to richen the fuel mixture (large vacuum leak indicates to the MAP sensor that the throttle blades are wide open or at WOT. If the MAP sensor sees this, it causes the mixture to go full rich to protect the motor. A lean condition at WOT can cause the aluminum pistons to burn up, so the fuel injection causes the motor to go full rich). This makes the engine smooth out and idle correctly.

This doesn't necessarily mean that the MAP sensor is bad in your case. The vacuum leak you introduced at the MAP sensor caused the mixture to go rich, so it is at least partially working.

Make sure the vacuum lines that go to the MAP sensor aren't hard at the ends or cracked. If the ends are hard, cut them off and reinstall them or replace them entirely. Check the vacuum line going to the PCV valve for cracks or leaks.

Check or replace the CTS, next to the thermostat. This controls the fuel mixture to a great extent. IF this is the temp sensor you replaced (there are two, so we always have to be sure of WHICH sensor you replaced), double check the operation of this part. Never assume that new parts are correctly working parts.

You test this part using a non-contact infrared thermometer, and compare it to the resistance reading in the CTS. Use this table to check: sensor...

Make sure your thermostat is a 195 degree thermostat. All US domestic auto manufacturers use 200 degrees F as their standard temperature, and the fuel injection systems are designed to use that temperature for greatest efficiency. If the temperature is less than that (like 180 or 160) the fuel injection will have the incorrect fuel mixture.

Check the spray out of the injectors and the fuel pressure as suggested. Incorrect fuel pressure will change the air/fuel mixture.

Forum Newbie
Posts: 4

Reg: 03-09-17
03-15-17 11:53 AM - Post#2681644    
    In response to CowboyTrukr

Thanks everyone I'll try these things this weekend. Waiting for weather to break cold and snow last few day. I was beginning to think about fuel pump but this helps to rule out these things first
Thanks again PClout

Forum Newbie
Posts: 2

Reg: 03-20-17
03-20-17 09:10 AM - Post#2682385    
    In response to PClout

I'm in on this post with a 1990 Suburban 5.7 4x4. that will idle but chugs heavily and dies when you try to accelerate. I'll also go check the fuel pressure and fuel spray. Maybe we can all help eachother here.

1990 Chevy V1500 Suburban (4x4)5.7L
1995 Chevy K1500 (4x4)5.7L
1976 Chevy 2500 Flatbed 4x4 454ci
2005 Chevy 1500 (4x4)5.3L
1999 Ford f350 Super Duty Dually 6.8L V-10
1969 M35A2 Deuce and a half turbo diesel
1997 Toyota 4Runner 4x4 3.4L

Posts: 176
Age: 33
Loc: San Antonio, TX
Reg: 09-15-10
03-20-17 01:39 PM - Post#2682416    
    In response to rotorav8

My 91 did the same surging at idle that you have. The culprit on mine was the small piece of rubber hose that connects the fuel pump outlet in the tank to the metal tube that goes up and out of the fuel tank. The hose had split lengthwise because it was not fuel hose and half the fuel going through the pump was spraying back into the tank so I had low fuel pressure.

91 Chevy C1500 ext cab, TBI 350, NV4500, 2in suspension lift. (DD)
2012 Mitsu Outlander Sport (DD#2)

Forum Newbie
Posts: 4

Reg: 03-09-17
03-25-17 06:00 PM - Post#2683302    
    In response to tardisguy

Just wanted to say thanks again. Found the issue, it was the fuel pump only 5psi. Replaced entire sending unit and pump. Purrs like kitten again.
Thanks to all comments

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