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87 Suburban with surging issue

B

Bob87Burb

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I've been chasing this problem for a while, and can't seem to find it. The truck is an 87 Suburban 1500, 350 TBI/700R4. I've done the normal maintenance (plugs, wires, tune up stuff), checked for vacuum leaks, replaced all vacuum hoses, tried a different MAP sensor from a truck that ran well, checked TPS voltage, checked for a sticking EGR, checked fuel pressure, and timing. All of those things were fine, or adjusted to be within specs. The problem is, under acceleration, the truck surges slightly. It's not a huge surge that causes drivability issues or is a danger, but it's aggravating as hell. The truck is not setting the check engine light at all. I have not hooked it to a scanner though, I don't have one or access to one. I just checked the codes using the jumper method on the ALDL. I've been reading up on the knock sensor and its operation, and from what I gather when they fail, the truck will set a check engine light, and performance will suffer severely due to timing not being allowed to advance. It's hard to explain, but definitely feels like an ignition issue, more precisely timing. Could the knock sensor be intermittently failing, and not setting a CEL? What's a good way to check the knock sensor? Any other ideas? I do all the work to all my vehicles myself, and I'd hate to bring this in and pay a bunch of money for a simple problem I may have overlooked.

Sorry the post is so long, I wanted to include as much info as possible.

Thanks in advance!
-Bob
 
If I remember the knock sensor operation correctly, intermittently the ECM does something (advances timing a fair amount?) that is supposed to induce knock. If it doesn't see the knock, the CEL should come on.

A problem that is consistent but not constant, would lead me away from blaming the knock sensor.

Your knock sensor may very well be seeing a problem, and retarding timing, which is why it seems to surge. Of course, it could just as easily be an ignition problem of some sort...idle is perfect?

Are you sure the TPS actually works, and doesn't have "dead spots" in it?
 
Could be your knock sensor picking up knocking/vibration from the motor mounts if the mounts are old rubber and worn out. I was having a strange surging off and on checked every thing found old mounts were worn. Replaced with poly mounts and have not had a surging problem since. Just my .02
 
I am going to guess from my similar experience that you have a fuel pump going bad, the fuel pump will not show any codes either. You can disconnect the knock sensor and it shouldn't pull any codes. The kock sensors job is to report any pinging to the ECM so that the ECM can correct ignition timing accordingly. If the knock sensor is disconnected the ECM would never know since it only relies on an electrical signal that is given if there is knocking occurring.
 
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If I remember the knock sensor operation correctly, intermittently the ECM does something (advances timing a fair amount?) that is supposed to induce knock. If it doesn't see the knock, the CEL should come on.

A problem that is consistent but not constant, would lead me away from blaming the knock sensor.

Your knock sensor may very well be seeing a problem, and retarding timing, which is why it seems to surge. Of course, it could just as easily be an ignition problem of some sort...idle is perfect?

Are you sure the TPS actually works, and doesn't have "dead spots" in it?

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Idle is perfect, although I had to set it 50 RPM higher to prevent stalling when stopped at a light, etc. It's at 800 in gear. I thought the same thing, that it was picking up knock, so I tried 93 octane, it did not help.

I'll check the TPS for dead spots, though when I checked it before I didn't see anything. Wasn't really looking for that though.

I've read that the knock sensor/ecm is capable of pulling back 20 degrees of advance, and when it surges, thats how bad it feels. It's noticable.

I've read about weird things setting off knock sensors too, like loud exhaust, etc. I'll check the motor mounts, I know the trans mount is going bad, but I didn't check the motor mounts.

Thanks for all the help guys, keep it coming! /forums/images/graemlins/thumb.gif
 
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I am going to guess from my similar experience that you have a fuel pump going bad, the fuel pump will not show any codes either. You can disconnect the knock sensor and it shouldn't pull any codes. The kock sensors job is to report any pinging to the ECM so that the ECM can correct ignition timing accordingly. If the knock sensor is disconnected the ECM would never know since it only relies on an electrical signal that is given if there is knocking occurring.

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You make a good point about the fuel pump. It would also relate to the problem of the truck taking longer than normal to crank over when it's been sitting more than a few minutes. I'll check that as well. For reference, are there any upgraded fuel pumps available? I don't really need one now, but plans call for a cam swap, and most likely a Holley TBI. If I'm going to need a high volume pump, may as well do it now.
 
Hey my sub surges too. When accelerating, the higher rpms it has power steps very noticible notches. I am going to replace the knock sensor this weekend since this is cheaper than a new ECM. I am getting a code for the ECM but KS will effect the ECM performance. Ill let you know if it helps at all if you want /forums/images/graemlins/burb.gif
 
800 in gear sounds high to me, but I'm not sure. What is the spec on that engine?

If it's stalling at lights, you've DEFINITELY got something else going on. Unless of course idle was too low. 50RPM shouldn't make the difference though...I've watched my TPI dip as low as 400RPM and recover, so 50RPM difference around 800RPM certainly shouldn't cause an engine to stall.
 
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800 in gear sounds high to me, but I'm not sure. What is the spec on that engine?

If it's stalling at lights, you've DEFINITELY got something else going on. Unless of course idle was too low. 50RPM shouldn't make the difference though...I've watched my TPI dip as low as 400RPM and recover, so 50RPM difference around 800RPM certainly shouldn't cause an engine to stall.

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Spec is 750 in gear, but it would stall at that speed. Problem is, in Park, it's at about 1000. Sounds ridiculous, like a carb with a stuck high idle.
 
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Hey my sub surges too. When accelerating, the higher rpms it has power steps very noticible notches. I am going to replace the knock sensor this weekend since this is cheaper than a new ECM. I am getting a code for the ECM but KS will effect the ECM performance. Ill let you know if it helps at all if you want /forums/images/graemlins/burb.gif

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That's EXACTLY what mine does, though I don't have any codes at all. I hate to just start throwing parts at it (mostly because it's usually the last one you buy that fixes it /forums/images/graemlins/crazy.gif), but I may try that as well. I need to tow a car trailer from Long Island to RI on 7/5, so I'd like to get this sorted out before then. Thanks for the info!
 
Have you pulled and cleaned the IAC and it's passageways?

Seems to be a pretty common cause of idle issues, and if nothing else, is preventive maintenance.
 
Yep, it's actuall not that old, but I took it out and cleaned it anyhow.
 
My old 350 that was in my K5 did the same thing when the puel pump started going out..I would hook up a pressure gauge to the TBI feed line and see if you pressure is anywhere near 12psi as it should be...In my case, it was not. I was sitting about 9psi and it would drop as low as 6psi when I opened it up..That was with the 350..That dying pump would barely let the 454 idle and as soon as you got on the gas, it would fall FLAT on its face...new pump fixed my problems...

Chris
 
What about plug wires going bad. How old. If they are near the exhaust, they will go bad quickly.
Mine has headers. I put wires on it almost every year. Go to Auto Zone, get the lifetime wires, never pay again.
 
OK< here's an update. I checked the fuel pressure, and it seems fine, so I'm going to rule out the fuel pump. Another factor in that is that when it's wide open, power seems fine, with no hesitation or surging. This is really driving me nuts!

I'm going to go through the ignition tonight. Plugs, wires, cap and rotor were done last year, with all Autozone stuff. Maybe you're right detailer, the wires could be bad. At any rate, like you said, they're free for life. They're gonna lost money on this one. /forums/images/graemlins/rotfl.gif I'm going to do the plugs as well. I put the Bosch platinums in last year, but I've heard some bad things about them.

Also, the truck seems to skip slightly at part throttle as well. This is leading me to think there's either a vacuum leak, or it has a bad MAP sensor. Still not throwing any codes, but I guess it's possible.
 
Well I changed the knock sensor today. It doesnt ping under acceleration or give me a code but, it still surges /forums/images/graemlins/angryfire.gif And since I fixed one problem, naturaly a new problem must replace it now my clutch cylinder leaks /forums/images/graemlins/angryfire.gif /forums/images/graemlins/angryfire.gif Anyways I have replaced my wires, plugs, coil and cap with mostly MSD so thats not a likly problem. Gonna change fuel filter and cheak the presure /forums/images/graemlins/thinking.gif /forums/images/graemlins/dunno.gif
 
I'm sure there are multiple things it could still be, but just seems like EGR is still a very probable cause.

Have you unhooked the EGR from vacuum source and tested?

Obviously if the valve was always hung open, idle would be poor, and you shouldn't notice it under acceleration (no vacuum to open it) but under the right conditions it might open enough to cause the problems you are experiencing.
 
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I'm sure there are multiple things it could still be, but just seems like EGR is still a very probable cause.

Have you unhooked the EGR from vacuum source and tested?

Obviously if the valve was always hung open, idle would be poor, and you shouldn't notice it under acceleration (no vacuum to open it) but under the right conditions it might open enough to cause the problems you are experiencing.

[/ QUOTE ]

I was thinking the same thing on the way home tonight. That's on my list of things to check as well. I know it's not hung open, but what could make it open when it's not supposed to?
 
*apparently* some of the designs (there are two) can be opened by exhaust pressure, so I've heard.

Other thing is that perhaps it IS sticky, but still operates smoothly...there is typcially so much carbon in the passageways, it is very possible for the valve to not seat well. Or at least I can see it happening.

Another thing, does the EGR valve actually hold vacuum? I don't know if your TBI trucks use an EGR solenoid, check it for a vacuum leak from the solenoid to EGR valve, if present, as well.
 
In my experience surging problems are most often caused by fuel delivery problems. Ignition problems will usually present some misfiring at idle or break down at higher RPM's. Vacuum leaks will also cause misfiring. If your engine is idling smooth, and has no problems at higher RPM's, then you should start looking at fuel system problems and not ignition problems. Bad fuel injectors, clogged fuel filters, collapsed fuel tank pick-up sock, and fuel pumps are the major suspects with surging problems.
 
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