92 Sonoma Sluggish When Warm

Hi;
I've spend the past day or so cruising the S15 forum past posts, FAQs and how-tos, but have not quite got a direction to help diagnose my problem. Hoping someone out there has experienced my situation before and can lead the way.

I have a 92 Sonoma Ext Cab 4x4 with 4.3l V6 and manual transmission, which I have owned since new. The truck has about 150,000kms on it and has given yeoman service with very few problems over the years. Recently, on a routine service visit, the mechanic noticed that the distributor cap was cracked and replaced it.

As often happens one of the cap hold-down screws was seized in the distributor and cracked off (happened to me several years ago as well). This necessitated removal of the distributor to repair. The distributor was marked, removed, the screw removed and the new cap secured...then the distributor dropped back into place. The timing was checked (more on this later) and all was well. The truck started, idled great and appeared to run great on a quick road test.

On the ride home, while the engine ran smooth as silk, the truck did not seem to have much power. Normal cruising in 5th now needed 4th with lots of accelerator or even 3rd...anything uphill at all was a challenge.

The truck starts just fine...idle seems a bit low, but about what it has been for the past 5-10 years. Cold drivability is just fine...lots of power and no stumbling of any kind. As the engine warms up, however, the same "low power" situation returns. Once the revs get in the midrange the engine seems fine, but anything under 2500rpm and the engine struggles to keep up, but falls just short of misfiring or bucking. The engine can rev to redline just fine in the lower gears. No hot/warm start issues...only this low power problem. Feels to me like no ignition advance in the lower rpm range when warm...but that's not very scientific.

I brought the truck back to the shop and had them look at it again. The tech verified on the 'scope that the timing was "OK". I did not know at the time about the ESC wire under the dash, so I don't know if this was disconnected when the original timing work was done or on the recheck. I plan to go back again and ask this question. It is connected now...and looks" like someone was at it, but that could have been any time in the past 15 years!

The truck ran fine before the dist cap R&R, so I feel pretty confident the problem is directly related to that work. The service engine light is not lit and I have checked the computer several times for codes...all clear everytime.

Any ideas what's up?

Regards,
SteveCo in St. John's

Since the computer controls the ignition timing after the initial timing is set and varies the timing several degrees to accomodate the knock sensor & the computer seems to "learn" what's normal...I'd disconnect the battery for a few minutes and make sure there's no cell phone/game chargers plugged in the power outlets. This will make the computer re-learn the running conditions and maybe get more active with the ignition timing.

Thanks for the suggestion. Makes sense and is easy to try. I'll give it a shot and let you know what happens.

Regards,
SteveCo in St. John's

Well, I disconnected the battery for about 45 minutes, connected it back up and went for a ride. Problem is still the same...no difference. Ran the truck for about 20 minutes and it did not improve that I can tell.

While I was at it I confirmed that the ESC was working. I disconnected the ESC wire (brown with white stripe wire under dash just above the passenger footwell) and the engine nearly died. Reconnected the engine recovered to idle solid, if a little slow...exactly the same as it has been for some time.

Any other ideas? I have read some posts where people with similar symptoms have disconnected the EGR valve and plug the vac line and gotten improvement. Might try that, but what does it mean if this works? How would I fix it permanenetly?

Regards,
SteveCo in St. John's

You might check your fuel pressure, mine is sluggish and I'm about 5psi off on my fuel pressure. Do you know whether yours is a TBI fueler (round air cleaner) or a CPI (square air cleaner box)?

Mine is TBI (The round air cleaner).

I have done a bit more seat-of-the-pants diagnosis and I think the problem may be more fuel related that ignition. There is definitely no rich mixture conditions (other than the first 5 seconds after a dead cold start) anytime...no black exhaust and the inside of the tailpipe has very little soot on it. Maybe the computer is getting wrong info and dialing back the mixture...or maybe the fuel is just not getting to the engine. I'll check the fuel pressure to confirm that part.

I am suspecting that the low idle is related to the sluggish performance. Basically there is no fast idle on my truck. Started from dead cold and it idles like it does when warm...you almost thiink the engine will die when cold idling, but it never does. I have not checked, but I looks like the idle is 500-600 rpm.

I'm thinking that the problem might be clogged injectors or faulty throttle position sensor. Anyone have the procedure to check the TPS for operation? Feel to me like the throttle plates open, but no additional fuel gets injected...unless you floor the throttle. Then things get a little better.

Someone else suggested checking the EGR valve for a vacuum leak...not sure if this would cause the problem I'm seeing, but it's worth a try. A number of wires and vacuum connections were disconnected to R&R the distributor, so it's a plausable source of the problem.

Regards,
SteveCo in St. John's

You might check the Idle Air Control valve (IAC), it's screwed into the side of the TBI and has a square 2 wire connector. After removing it, you can spray some carb. cleaner into the hole but not on the IAC itself. If the IAC is gummed up, clean it with a toothbrush and some mineral spirits or kerosene...you don't want to melt the IAC seal with strong stuff.

Thanks for the tip on the IAC. I wanted to look at that anyway.

Regards,
SteveCo in St. John's

Just a quick update on the above. I made up an ALDL cable and connected a laptop with WinALDL to the truck. All the sensors and readings seems to be just fine (as expected)...TPS, IAC, MAP, etc...all seems to provide the correct range of readings.

I monitored the engine from a full cold start to normal operating temp with WinALDL. Couple of things I noned was that the idle was a little erratic (moved between 650 and 750 rpm a fair bit), but it never came close to stalling. Warm idle is rock solid at 600rpm. Still wondering of the cold idle is a bit slow...just sound like it is to me.

I logged about 15 minutes of data during a test drive...uphills, downhills, intersection stops, idle...basically normal daily driving. I have been analyzing the data and can see anything that jumps out as wrong. The MLB readings hover around the 125-130 mark (no real adjustment being made so I am told) and the INT readings vary a bit more, but again center around the 128 mark.

Can anyone point me to a way to use WinALDL to help better diagnose my problem? My next step is to log a cold drive (where the problem is less pronounced) and a warm drive on the same route, then compare the results.

Is there a resource on the 'Net to help use WinALDL for troubleshooting?

Regards,
SteveCo in St. John's

If you check John's Knock Tables, you'll see where he gets knock counts that come and go indicating the timing is being adjusted by the engine computer.

http://www.myelcamino.net/aldl/

Is your knock table active when you "grab" a log file?
After going through my manual, a check of your EGR valve might be a good idea too.

Was the ESC connector unplugged while it was re-timed?

I've been watching this thread and it appears that a "goose chase" may be in progress ;). All of your problems started after the distributor work. I can't help but wonder about the timing... If the ESC wire was not disconnected when they re-installed the distributor, and adjusted ignition timing, all of the symptoms you are experiencing could occur.

All distributor caps are not equal. By that I am referring to the exact placement of the towers for each spark plug. Distributor cap towers can be a few degrees off one direction or the other, depending on the manufacturer of the cap. This will affect ignition timing the same amount of degrees, either advance or retard. It has the same affect as rotating the distributor to adjust the timing.

My suggestion would be first of all, make sure the rotor was also replaced with the cap, if not, replace it now. Some vibration dampers have two marks close to TDC. If you are not sure which mark to use, manually rotate the crankshaft until # 1 cylinder is at TDC, (visible in the spark plug hole). Mark the line on the damper that lines up with TDC on the index, with a white crayon or paint. Disconnect the ESC wire, check and adjust the ignition timing, and reconnect the ESC wire.

It's possible that the combination of these things affected ignition timing adversely and that's your only problem. Hope this helps.

I've been watching this thread and it appears that a "goose chase" may be in progress ;). All of your problems started after the distributor work. I can't help but wonder about the timing... If the ESC wire was not disconnected when they re-installed the distributor, and adjusted ignition timing, all of the symptoms you are experiencing could occur.

All distributor caps are not equal. By that I am referring to the exact placement of the towers for each spark plug. Distributor cap towers can be a few degrees off one direction or the other, depending on the manufacturer of the cap. This will affect ignition timing the same amount of degrees, either advance or retard. It has the same affect as rotating the distributor to adjust the timing.

My suggestion would be first of all, make sure the rotor was also replaced with the cap, if not, replace it now. Some vibration dampers have two marks close to TDC. If you are not sure which mark to use, manually rotate the crankshaft until # 1 cylinder is at TDC, (visible in the spark plug hole). Mark the line on the damper that lines up with TDC on the index, with a white crayon or paint. Disconnect the ESC wire, check and adjust the ignition timing, and reconnect the ESC wire.

It's possible that the combination of these things affected ignition timing adversely and that's your only problem. Hope this helps.

I agree, the ESC wire needs to be disconnected while making any adjustments to base timing. If you try to adjust timing while the ESC is not unplugged, you will have problems similar to what you are saying, because base timing is not correct, and all timing stratgies the ECM uses will always be off/not right. The ECM needs proper base timing. Warm the truck up, disconnect the EC bypass wire, and then set base timing. Clear codes, reset computer and go from there.

Thanks for the suggestions. And yes, I agree that timing seems to be the most logical source of the problem. The shop that did the work (and checked it a 2nd time) replaced both the rotor and cap...and the mechanic confirmed he disconnected the ESC wire when setting the timing. This shop uses GMC trucks for their mobile fleet and have for 20+ years...so, I'm 95% sure they did things right. I do plan to get the timing checked (I don't have the equipment to do it myself) by another shop to confirm. The vehcile is not in constant use this time of the year, so it's not a pressing issue right now.

Beyond this issue, however, I do wish to learn a bit more about problem diagnosis via the engine computer. Seems like a good time to do so.

Toward that end, I'm not seeing any knock counts on the computer. Might this be confirmation that the advance is not working correctly?

Regards,
SteveCo in St. John's

The reason I thought it was timed wrong is when you said you unhooked the ESC conn. and the engine died. That sounds like the timing is retarded quite a bit.

I did disconnect the ESC wire and the engine stumbled, but did not die...probably would have if I left it that way for long, but I reconnected after about 10 seconds.

Someone else (not on this board) suggested I try disconnecting the ESC wire at idle to confirm that spark advance was actually happening. At least I know that some advance is occuring...but the orginal point you made is valid: I need to confirm that the base engining timing is set correctly.

I'll try to do that on the weekend (travelling thsi week) and will let you all know what I find.

Regards,
SteveCo in St. John's

Keep us posted... I get to travel 30 miles in snow/ice to work this morning.....

I did disconnect the ESC wire and the engine stumbled, but did not die...probably would have if I left it that way for long, but I reconnected after about 10 seconds.

Someone else (not on this board) suggested I try disconnecting the ESC wire at idle to confirm that spark advance was actually happening. At least I know that some advance is occuring...but the orginal point you made is valid: I need to confirm that the base engining timing is set correctly.

Steve,

If the base timing is set correctly, disconnecting the EST bypass should NOT kill the engine. If nothing else was done but disconnecting the bypass connector, the ECM will revert to base timing, and the IAC should compensate for any RPM change and maintain the target idle RPM. The fact that the "engine stumbled" may indicate two problems. The first may be the base timing setting. The second may be the IAC function. Don't replace the IAC unnecessarily. Since you have to remove the IAC to replace it, you might as well just clean and relubricate the existing IAC and install it. While it is out, clean the IAC air passafges thoroughly. That would have to be done with a new IAC anyway. You also need to check the throttle minimum air position adjustment, and before doing that, clean the throttle body bores and plate thoroughly. Again, that should be done with even a NEW IAC.

The root problem sounds as if the timing was set without bypassing the ESC, so it is retarded by about 20º at idle. However, since the ECM could not maintain idle RPM, the throttle body and IAC likely needd attention.

You'll need to get your hands around a timing light, handheld tachometer, a large spray can of carburetor cleaner, and a couple drops of clean engine oil.

Update:
The weather here in the Far East of the Western World finally warmed up enough for me to investigate my problem a bit more. I did check the timing and found it to be retarded a fair bit, so I went thu the process of adjusting it. Rotating the distributor counter clockwise a bit produced a far smoother idle. When I started (ESC wire disconnected of course) the adjustment, I could not see the timing mark of the balancer. As I rotated the distributor a bit, I could see it was to the right of the timing pointer, so I kept rotating and checking the position of the timing mark.

I did have some question on what timing mark to use for the adustment. Since I can't post the picture directly on this forum (and haven't got a place at hand to do so elsewhere), I'll describe what I have and hope someone can straighten me out.

The timing pointer on my truck (when viewed from the front of the engine) sort of looks like this (appologies for the crude diagram...the hyphens to the left are just there to make the diagram work...please ignore them otherwise):
--\___/\
---VVV |___|

I'm assuming the inverted "V" notch in the middle is 0deg BTDC and the sawteeth to the right of it are successive degrees of advance. Can someone confirm if this is indeed correct?

Right now I have the timing set a bit advanced...the timing mark is aligned with the middle sawtooth. I am getting a small bit of pinging at low RPM under load, so I'm guessing this is something like 3-4deg advance? Maybe each tooth is 2 deg?

I test drove the truck a couple of times and it now is back to the way I remember it...lots of power and no lugging in the higher gears. I plan to recheck the timing soon and back it off a bit to eliminate the pinging, but would like to confirm the reading of the timing pointer before I do this.

Before I started this process, I did check the timing at idle with the ESC wired still connected (I know, I know, that not the right way, but I was curious). What I found is the the timing was just about "right" at 0deg BTDC. It did move around a bit, but was clearly centering around TDC. So my guess is that the mechanic that R&R'd the distributor likely did not disconnected the ESC when he checked the timing. Either way, the base timinig was not set correctly. The truck runs great when everything is setup properly, as it is after my work yesterday. I have to fine tune it, but at least I know what the problem was.

So is 0deg BTDC the "big" inverted groove? What is the approximate value of the pointer teeth? Inguiring minds want to know...

Regards,
SteveCo in St. John's

It appears the first timing mark (The large"V") would be 0 degrees, and each mark is 2 degrees, and there are 4 degrees between each "V". It's sounds like the timing could be retarded another degree or so. If it's pinging now, wait 'til it's warm outside while your pulling a hill. You've got it pretty close, though.

I had a chance to run the truck for awhile yesterday evening and it seems to be fine as is. But you're right, the timing should be retarded a bit. While the weather here in NF is "temperate", the summer will bring much warmer temps than mid March. I will be re-adjusting the timing later in the week with a target to get the base timing to 0deg BTDC or just slightly advanced beyond that. I have to reset the computer to clear the ESC disconnect codes anyway.

It's good to finally have this source of this issue identified. As some have pointed out in this forum, the cause was the distributor R&R and subsequent (mis)check of the ignition timing. A friend mentioned yesterday that likely the technician involved put the engine on his computer analyzer after the repair and all looked fine. Since the computer did not indicate the timing was off, he assumed it wasn't. I doubt he pulled out the timing light to verify.

Likely, he did not realize that this era of vehicle has no specific sensor for timing. This jives with my recollection of the 2nd visit to the garage...a 20-something tech did the recheck of the distributor work...probably just did not know the timing had to be verified manually! Live and learn!

Anyway, now I know a lot more about the vehicle I have owned for 15+ years...and I have the computer diagnostic bits and pieces now fix things if and when they go wrong. Thanks to all who offered suggestions and ideas.

Regards,
SteveCo in St. John's