Author Topic: Stalling when barely pressing the throttle  (Read 4950 times)

Offline Jon87V20

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Stalling when barely pressing the throttle
« on: July 16, 2017, 04:29:19 PM »
Hi fellas,

When I bought my '87 Suburban V20 (350TBI/TH400/NP205) a few years ago, after long drives (an hour or so) on the highway it would idle really low after coming to a stop. Probably around 400 or 500rpm. If I didn't do anything, it would usually keep running, but just barely until I started moving again. If I came to a stop and then went in to reverse, it would usually die right when reverse engaged. It restarts fairly quickly after dying, then the idle is high until I've been driving again. About 1,200rpm

As of the past month or so, this has gotten a bit worse and now there is a new symptom as well. Now if I'm barely pressing the trottle (imagine how much you press the throttle when looking for a parking spot at the grocery store, or just through your neighborhood when coming home, just enough to keep the truck moving) it will misfire randomly on all cylinders. If I let up off of the throttle, it clears up. If I give it more throttle, it hesitates momentarily and then picks up and goes just fine. It only does that in that one spot in the throttle range. Engine speed or load don't seem to affect it.

I've also noticed that now sometimes it will randomly have a higher than usual idle (about 1,100 to 1,200rpm) regardless of what's been going on.

Things I've changed since buying this truck are, transmission, air filter, spark plugs, lower radiator hose, battery, starter, I removed the AC pump as it wasn't working, and I got rid of the factory Y-pipe and put in a dual pipe setup I found in a junkyard Suburban. Also what I believe would've been the smog pump or air pump (not sure what it's called) was removed by the PO at some point because the bracket for it is just empty. There was some tubing that I think used to connect to the smog pump that runs above the exhaust manifolds on either side and I removed it from the passenger side because it was blocking the spark plugs and was pissing me off. I cut it out of there.

Some things of note: This truck has started burning a lot of oil (probably about a quart a week with quite a bit of operation) and there's noticeable blue smoke at idle. There's a pretty hefty exhaust leak at the collectors on both sides because I had to drill the studs out and I replaced them with some grade 8 hardware to get me by for now but they've loosened up a bit and I haven't been under there to tighten them up as of late. I had the truck up at a very high elevation and was running it in 4L in first gear to get through some sketchy terrain and the Service Engine Soon light came on while the rpms were high up during engine braking coming down the hill. As soon as it returned to idle, the light went out. This happened twice that day under the same conditions.

I found a few small vacuum leaks and plugged them up. I can't seem to find or hear any other leaks.

My question is, what would be causing it to miss and run weird just in that position on the throttle? It's very irritating when I have to keep feathering the gas to keep it running in the parking lot all the time. I plan to rebuild or replace this engine within the next few years but I need this to work as a daily driver for the foreseeable future and want to keep it as happy as I can.

Thanks!
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline dwightr

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Re: Stalling when barely pressing the throttle
« Reply #1 on: July 16, 2017, 04:51:31 PM »
There are 2 things I would investigate.#1 throttle position sensor and #2 would be the idle air control valve they do cabin up and not operate properly as for the oil burning it sounds like possibly valve seals. I hope this helps.

Offline bd

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Re: Stalling when barely pressing the throttle
« Reply #2 on: July 16, 2017, 05:30:36 PM »
Is this on your 1987 V20 w/5.7K engine?  First, check the ECM for trouble codes and record them.  Check the fuel filter and measure fuel pressure using a suitable gauge.  Remove and inspect the spark plugs, keeping them in order by cylinder.  Post pics of the plugs if you can.  Check the torque on the intake manifold bolts using the factory torque sequence found in the service manual.  How many miles are on the engine?  Drop some oil from the dipstick onto a clean bare fingertip - does the oil maintain a tight circle or does it quickly track out along the finger ridges?  With the engine off, open the throttle about about 25% and wiggle the throttle shaft side-to-side.  Is there more than ~0.010" of side play between the shaft and its bore?  With the engine running, spray some B12 carburetor cleaner around the base of the throttle body.  Is there any change in engine speed?  Check the torque on the throttle body-to-manifold bolts.  Temporarily unplug the vacuum hose from the EGR valve and drive the vehicle around.  (If the Check Engine warning illuminates with the EGR disconnected, disregard it for the time being.  In addition, the engine may ping under load with the EGR disconnected).  Are there any changes in symptoms? 
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Jon87V20

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Re: Stalling when barely pressing the throttle
« Reply #3 on: July 16, 2017, 09:19:38 PM »
Yes, this is regarding the '87 V20 with a 350 tbi. Will the ECM still store codes even if the light is no longer lit?
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline bd

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Re: Stalling when barely pressing the throttle
« Reply #4 on: July 16, 2017, 09:57:20 PM »
As long as battery power to the ECM has not been interrupted (the battery has not been disconnected and the ECM B fuse has net been removed), a diagnostic trouble code will remain in memory for 50 consecutive ignition cycles following the most recent setting of that particular code.  If the problem that triggered the code does not recur within 50 ignition cycles, the trouble code will be erased from ECM memory, automatically.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline SkinnyG

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Re: Stalling when barely pressing the throttle
« Reply #5 on: July 16, 2017, 11:09:44 PM »
+1 on the Throttle Position Sensor.  Since your issue sounds not vehicle-speed or engine-speed but throttle-position related, that's the tree I would bark up first.

I'm also with dwightr on looking at the IAC next if that doesn't fix it.
The Crusty Chevy: 1977 Silverado, shortboxed & dropped, potato-potato.

Offline Jon87V20

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Re: Stalling when barely pressing the throttle
« Reply #6 on: July 17, 2017, 07:27:07 PM »
Oh ok. The code is probably lost. The battery terminal was loose last week and it wouldn't start until I tightened it. I'll try the troubleshooting suggestions listed here asap and report back with my findings. Thanks, guys.
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline Irish_Alley

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Offline Da67goatman

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Re: Stalling when barely pressing the throttle
« Reply #8 on: July 19, 2017, 03:53:46 AM »
I have had similar problems on different engines, one of them being stock and one of them being built.  I think I have narrowed it down to a tuning related problem as when using either of my "hot" chips this problem went away.
<Pitcrew>
1989 R2500 Suburban mild 350 TBI 700r4 10.5" 14bolt 4.10 w/ 33s
1992 fullsize Blazer 4x4  35s no lift, 4.10s, Hella driving lights, TJ Flares, Huge bumpers, Snorkel, custom interior

Offline Jon87V20

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Re: Stalling when barely pressing the throttle
« Reply #9 on: March 12, 2018, 05:56:48 PM »
        Ok. Sorry it took six months to update this, BD.

        There are no ECM trouble codes

        I don't have a fuel pressure tester right now so I haven't checked that.

        I had removed all the spark plugs and had them in order. However they got jumbled up during my exhaust project. Here's a picture of some of them. They all looked the same. They all have a burnt looking spot on them.

        The intake manifold bolts are tight

        The odometer is showing 83,xxx. Not sure if that 183,000 or 283,000

        When I dropped some oil on my finger, it maintained a tight circle and did not run out to the edges.

        As you can see in the video, it seems like there's quite a bit of side-play in the throttle.

        No change in engine speed was detected when spraying carb cleaner around the edges of the TB.

        The bolts connecting the TB to the intake manifold were surprisingly loose. I tightened them down.

        I haven't tried driving with the EGR unplugged yet. However, I did unplug it while it was idling and I didn't notice any change.

        I did notice when I hold the throttle open in the problem position while watching the injectors, it almost sounds as though they stop spraying evenly. It almost sounds like there was air coming through intermittently. That could just be me though...

https://www.dropbox.com/s/pnq4dgv963s95cn/TBI%20Play.MOV?dl=0[/list][/list][/list]
« Last Edit: March 12, 2018, 06:02:30 PM by Jon87V20 »
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline bd

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Re: Stalling when barely pressing the throttle
« Reply #10 on: March 12, 2018, 10:27:15 PM »
You still need to verify fuel pressure.  Do not neglect this step as it is fundamental to TBI function.  You need to road check with EGR disabled as well.

Wiggling the throttle shaft from end-to-end (i.e., the linked video) is checking axial play.  Because of the circular shape of the throttle plate and the small cavities created by the throttle shaft bore, you will always observe some amount of axial play in the shaft with the throttle open.  That is, some play is normal increasing with throttle opening.  But, excessive axial play will result if the throttle plates are edge worn.  In addition, there can occur excessive radial play in the throttle shaft if the throttle shaft bore is worn.





Referring to the image, the left illustration exemplifies zero wear and is optimum. 

In contrast, the right illustration shows causes for both axial and radial play.   

With the engine off, look down the throttle bores using a bright flashlight and inspect around the edges of the throttle plates with the throttle open ~5° while wiggling the throttle, initially side-to-side (front-to-rear as illustrated at the right margin of the image) and then end-to-end (as you did in the video). 

Excess "axial play" is caused by throttle plate wear at opposite sides of the throttle plate where the plate intersects the throttle shaft.  The throttle plate is worn noticeably narrower resulting in visible gaps between the plate and its bore in the area of the throttle shaft with the throttle almost closed.  This allows the shaft to move excessively along its axis - the plate does not remain centered in its bore.  If you observe throttle plate side wear as shown in the right illustration, the throttle plate is worn out and idle speed stability may suffer.

Excess "radial play" is caused by throttle shaft bore wear.  Shaft bore wear will cause the "big gap" illustrated at the front and rear of the plate, between the throttle plate and its bore as you wiggle the shaft front-to-rear.  If you observe excess radial play, the throttle shaft bore is worn out, resulting in poor idle speed control and erratic TPS base adjustment.

Either of the foregoing faults calls for TB overhaul or replacement.

Use a DVM and check TPS voltage.  Make up a temporary three-wire jumper so you can measure TPS voltage without piercing any wire insulation - a couple of Weatherpak connectors and the appropriate terminals will do the trick.  Measure the voltage change as you slowly open and close the throttle with the ignition on, engine off.  There should be a very consistent change in voltage with no erratic jumps.  If the voltage is erratic, replace the TPS.  You can use an ohmmeter for this test, but I do not recommend it.  An ohmmeter can indicate a false fail of a perfectly good TPS.

In addition, check the MAP sensor using the criteria linked by Irish_Alley.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 805.K10

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Re: Stalling when barely pressing the throttle
« Reply #11 on: March 13, 2018, 06:59:27 PM »
If I recall GM OBD 1 doesn't always record intermittent errors/issues. The fault has to continue for several minutes before the TC is recorded.

Offline Jon87V20

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Re: Stalling when barely pressing the throttle
« Reply #12 on: May 21, 2018, 10:51:12 PM »
So I think I may have figured out what the issue was. I went to change my cap and rotor last weekend and I noticed the distributor wasn't tightened down very well at all! It was able to move fairly easily.  Today I went out and set the base timing and zero and it's running WAY better. It doesn't seem to be stumbling under light throttle anymore. I'll know for sure tomorrow because I'm taking it to work. Seems much more happy now though.
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208

Offline Jon87V20

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Re: Stalling when barely pressing the throttle
« Reply #13 on: July 05, 2018, 06:10:37 PM »
This actually turned out to be a failing ignition control module for anyone else that's looking for info on this. It started getting really bad until it was throwing a code 42. After changing the ICM, the hesitation seems to be gone!
1987 Chevrolet Suburban V20 Silverado
350ci TBI TH400 NP208