1997 Suburban 5.7L rough idle / stumbles during acceleration

[billyrb]

I just replaced the motor on a suburban I bought. When I bought it, the original motor was shot so I never got to hear it run. It idles and runs / drives, but there is a stumble both at idle and while accelerating. There is noticeable power loss, but it gets less noticeable when the rpm's get higher. Sometimes at idle you can barely tell, other times it's very noticeable. I checked the plugs and wires, and they are good. New distributor / cap / etc. I'm wondering if anyone has any suggestions?

On the passenger valve cover is a Purge Vac solenoid, and it has a line from the Charcoal Canister that connects to it, and a vac line from it to another solenoid back by the EFI electrical connector. But, there is a vac line on the side of that front-most Purge Vac solenoid that has no vac line connected to it.....anybody know what goes to this port?

Also, do the standard OBD-II scan tools pick up stumbles / misfires? Do they also provide real-time scans, or only pull codes from the PCM? I was thinking about taking it to the local AutoZone to borrow one of their scanners.....

[Trevor350]

you can get it scanned there.

Is fuel pressure good?
Any vaccum leaks?
Timing my also be off so I would check all these things and go from there. Of course get scanned for codes first.

[98GreenMachine]

Check the fuel pressure. The test port is on top the engine on the fuel line. With the key on, engine off, pressure should be 60-66 psi.

[billyrb]

I'm going to buy a fuel pressure test gauge this weekend. For vacuum leaks, I found the one port (mentioned above) that doesn't have a line connecting to it, but I wasn't sure if that port was to vent to the air or not (can't find any diagrams or images of it anywhere).

For timing, I checked under the hood and it doesn't say what it should be (Vortec engine).....only says timing is controlled by the computer, yadda yadda yadda. Does anyone have the spec?

[98GreenMachine]

Originally Posted by billyrb

I'm going to buy a fuel pressure test gauge this weekend. For vacuum leaks, I found the one port (mentioned above) that doesn't have a line connecting to it, but I wasn't sure if that port was to vent to the air or not (can't find any diagrams or images of it anywhere).

For timing, I checked under the hood and it doesn't say what it should be (Vortec engine).....only says timing is controlled by the computer, yadda yadda yadda. Does anyone have the spec?

On the 96+ engines, you cant adjust the timing the old way with a timing light. You would need a scanner that reads the Camshaft Position Sensor retard value. This value must be +/- 2* off of 0*. You adjust this by turning the base of the distributor slightly one way or the other. The camshaft position sensor in conjunction with the crankshaft position sensor is what determines the timing on these engines.

[Torquer]

if i had a vacuum line i didnt know where it went i would at least plug it up with something, so you dont loose vacuum pressure. If there is a vacuum hose on it try to see where it reaches to, but if no line then um i dont know, try to get a diagram for your engine. i had a vacuum line on my old car that I found a screw that threaded into it perfectly.

for a rough idle maybe check the IAC plate area, make sure its well cleaned. make sure the IAC plate isnt sticking, usually does this in temperature extremes hot or cold. Then maybe check the TPS sensor, controls fuel. But this usually only affects idle and not so much when you get going. usually goes more on MAP and MAF sensors at driving speed

if its a weird looping idle, could be the MAF sensor or the o2 sensor.

If the engine runs rough at High or low speeds but fine in between. might be emissions sensors like EGR, EVAP, or PCV.

oh yeah and dont forget to check the air ductwork, that could caus an idle problem too. With some of the sensors to test them just unplug them before you start up the truck. The computer has a stored idle code in it if one of the sensors is unplugged it will just use the default code. so if the engine runs better without the sensor than you know where the problem is. this doesnt work on all vehicles though, some wont start with a sensor unplugged. and dont unplugg it with the engine running, that will usually give you a trouble code and a check engine light.

[billyrb]

it was what I thought.....dizzy issue. Got code P1345, and dizzy is one tooth off causing the rough idle, stumbling, etc. I'll remove it and try one tooth clockwise tomorrow night....if that doesn't work, I'll go back counterclockwise. That should fix it.

[98GreenMachine]

Originally Posted by billyrb

it was what I thought.....dizzy issue. Got code P1345, and dizzy is one tooth off causing the rough idle, stumbling, etc. I'll remove it and try one tooth clockwise tomorrow night....if that doesn't work, I'll go back counterclockwise. That should fix it.

Do you have your base/cap rotated alot one way? If your distributor cap is in its normal position, im surprised its even running with being one tooth off.

[billyrb]

base / cap can't be rotated on these, as they have specific screw holes to hold the cap firm, and they can't rotate like other caps. One tooth off could still run with the computer providing TONS of adjustment, but not enough to have a smooth running rig.

[98GreenMachine]

Originally Posted by billyrb

base / cap can't be rotated on these, as they have specific screw holes to hold the cap firm, and they can't rotate like other caps. One tooth off could still run with the computer providing TONS of adjustment, but not enough to have a smooth running rig.

I meant if the entire distributor was rotated one way or the other. When I was reinstalling my intake manifold I had the distributor one tooth off, and the only way to even remotely make it run was to rotate the base of the distributor about 20*. If I left the base/cap in its proper location, my truck wouldn't even start, just misfire. I pulled the dist out, turned it one tooth, aligned the base properly and it ran smooth as silk.

If you have the distributor one or two teeth off and the base is aligned properly, the engine can backfire or crossfire..which is rather dangerous.

[billyrb]

Well, I removed / reinstalled the dizzy one tooth in each direction, wouldn't run at all. Put it back where it was, ran but with same problem. I'm thinking the timing is still off and needs to be adjusted. Going to take it to the shop and have them run a full dianostic and figure out what's wrong.

[98GreenMachine]

Originally Posted by billyrb

Well, I removed / reinstalled the dizzy one tooth in each direction, wouldn't run at all. Put it back where it was, ran but with same problem. I'm thinking the timing is still off and needs to be adjusted. Going to take it to the shop and have them run a full dianostic and figure out what's wrong.

The timing chain could be loose or the cam/crank aren't aligned properly. A worn distributor gear can also cause some bad timing issues.

[billyrb]

all of them are brand-new, and all were aligned correctly prior to installation

[flylear45]

I am watching this with interest, I have the same issue. I bought a new scanner with realtime data only to find it still doesn't read the necessary data.

[AreikUSA]

Same here, i have a 96 1500 Truck 2WD With 5.7, i have it at -20 just to get it started and even then will not start right up takes forever almost to the point of killing my Battery. When i do get it started it has alot of low end torque but after it hits speeds of 35 to 40 it losses all power and becomes bogged down.

[Trevor350]

Originally Posted by AreikUSA

Same here, i have a 96 1500 Truck 2WD With 5.7, i have it at -20 just to get it started and even then will not start right up takes forever almost to the point of killing my Battery. When i do get it started it has alot of low end torque but after it hits speeds of 35 to 40 it losses all power and becomes bogged down.

Have you tested your coil?

[AreikUSA]

New coil, it was doing it before i changed it

[palpd96]

I am having same problems with my 96 5.7 vortec. Any solutions found?

[AreikUSA]

My problem turned out to be timing chain and distributer gears.

[70sbudget]

my 94 1500 had a similar problem. turned out to be a clogged cat. worth checking into. engine could not get rid of the exhaust properly. screwed EVERYTHING up. would not even run past idol.