problem with a slight miss and other odd issues

[sdelmar]

Hello all! I am new to the forum, but I have read several threads over the last few months. It looks like there are a lot of knowledgeable people on here, so I thought I would join and get some advice!

I have a 95 Sierra K1500 350/4L60E. I recently put a GM crate motor in after sucking a valve at 400,000 miles (my fault, I presume, after letting it overheat while rushing to the hospital on a bad radiator). Anyway, it has a slight miss at idle that can also be noticed at constant speeds around town (lower RPM). It has new plugs and wires, distributor checked out well with no play in the shaft, new cap and rotor, coolant temp sensor is working well and I have no exhaust leaks...at least that can be heard over the engine. It also runs very sluggish when cold while in closed loop and occasionally hesitates, almost killing the engine, when I hammer the throttle from idle. I have been getting a "lean exhaust" code that comes and goes, but most commonly comes on when driving at a constant speed. O2 sensor is working, vacuum is strong and I have a new fuel pump and fuel filter. When I replaced the fuel pump apparently a hose was not attached well because fuel spills over on fill-ups. To the best of my knowledge I don't think this should cause a problem on a low pressure TBI setup, but I'm not sure of that.

I have not checked fuel pressure, but the injector spray seems to be strong. Things I have on my short list to check out are:

1)map sensor
2)fuel pressure
3)ignition module

Sorry about the lengthy post, but I wanted to make sure I included all the info I have. Thanks in advance for your help! I am getting frustrated with this problem that I continue to throw money at!!!

[sdelmar]

One more quick thing: I have about 6 degrees of timing in it, but from what a couple of reputable mechanics have told me it should be no problem (and likely better power) on this engine.

[ChevLoRay]

TBI engines seem to have a plethora of issues. Search thru the threads and you'll find a bunch. Within those threads, you'll also find trouble-shooting tips and techniques.

[sdelmar]

replaced map sensor, and the problem is still there. Anyone with any suggestions? Has anyone else had a similar issue?

[ChevyTech]

Test the fuel pressure.

It does not matter if the pump is new.

There are other problems that can cause low fuel pressure other than the pump.

What brand of pump did you put in it?

If it was an aftermarket fuel pump that was installed, I would surely test the pressure.

I have seen so many pumps from other manufactures fail in the warranty period, or just after the warranty is up, that I suggest only using OEM (Original equipment manufacturer) fuel pumps.

The lean exhaust code also points to low fuel pressure.

[sdelmar]

I tested fuel pressure at the filter and I have 13-13.5 lbs with the key on/engine off and 14.5 lbs while running. Could it be that the fuel pressure regulator is bad?

I also noticed while I was under the truck today that I have a faint rattle in the catalytic converter. I am wondering if this might be restricting exhaust flow enough to cause problems.

Thanks for the advice ChevyTech. That at least eliminates one more possible problem!


I think I will try a different distributor next just to make sure mine is OK.

[ChevyTech]

A rattle in the catalytic converter is not a good sign, but being you said the “vacuum is strong” the exhaust should be ok, if you ment you put a vacuum gauge on it with the engine running and the vacuum is good.

If the exhaust is restricted, and you put a vacuum gauge on the engine, the vacuum should drop as you slowly increase the engine speed (RPMs), dropping lower and lower.

When you installed the engine and set the ignition timing did you unplug the set timing connector when you set the timing? If not the timing will be retarded from where it should be.

[sdelmar]

I did check with a vacuum gauge, but I did not check anywhere other than idle. The timing wire was disconnected (if I remember right, it was a wire under the dash on the passenger side) to set the timing.

[ChevyTech]

I see I didn’t answer your fuel pressure question.

14.5 PSI is out the top end of specification. Thing is, that by your description it sounds like it is more likely lacking fuel, rather then getting to much. The gauge could just be a little off.

The set timing wire is under the passenger side of the dash on a 1995 so it sounds like it was done correctly.

At this point I see nothing in the thread that leads me to a conclusion on what the problem is.

[sdelmar]

Yeah, it is frustrating!!!

I am hoping at this point it is a bad distributer, but I think it is unlikely! The thing that really has me concerned is that it runs so bad when it is cold. It dies almost every morning when I pull out of the garage, then it accelerates like it has a 10,000 lb trailer behind it until it warms up and comes out of closed-loop. What really throws me is that while it is cold and running so poorly, you can give it more throttle (about 3/4 throttle or more) and it improves, but I really don't like doing that to a cold motor.

[ChevyTech]

Actually open loop is when it is cold, and closed loop is when it is warm and using the oxygen sensor to adjust the mixture.

Did you test the coolant temp sensor with an ohm meter and specs, or do you think it is ok for another reason. The dash gauge and computer use two separate temperature sensors.

If you had a scan tool on it that could read sensor data we could see what the system is doing to make it run better when warmed up.

[ChevyTech]

When running cold (in open loop) the system is very dependant on the coolant temperature, TPS (throttle Position Sensor), and the MAP (Manifold absolute Pressure) to calculate the fuel needed to run well.

When you replaced the engine did you spray down the TBI unit getting carburetor cleaner on the TPS?

[sdelmar]

Ok, backwards on the open-loop/closed-loop! Coolant temp sensor was checked with multi-meter and was within spec. We also used a Snap-on Modis to verify temp showed on gauge and also shows everything else is operating properly. The dash gauge is off a little bit, but close.

It is very possible I might have gotten cleaner on the TPS! I have an extra one sitting around and will try that tomorrow. I don't have any bucking or jerking issues like I have experienced with a bad TPS before, but I presume a bad TPS might cause these issues?

Thanks again for all of your input! I know just enough to really get me in trouble and often cost myself more than just taking it to someone, but I still enjoy working on my own vehicles...especially when they are like Tinker Toys (TBI 350 is MUCH easier to work on than my Acura TL!!!!)!

[ChevyTech]

If you get a chance to use the Snap on scan tool again check the:

Block learn or BLM block Learn multiplier now known as is long term fuel trim

Integrator 0r INT now known as short term fuel trim

Check these after it has run enough to get warm and write down the numbers.

This will tell me if it is adding fuel or take fuel away to make it run better.

You could easily test the TPS and Map readings also.

[ChevLoRay]

The Snap-on scan tool is awesome! My cuz has one ($4K) and it shows stuff that is suspect, even if the Check Engine light isn't on. For example (and this was on my son-in-laws' 98 Dodge 4WD), there were two items that it displayed. One was the TCC (torque convertor clutch); the other was the CKS (crankshaft sensor). Replacing the crank sensor fixed the Dodge and the TCC hasn't been an issue. If you have an OBDI vehicle (pre 96), they are the tool to have access to.

[sdelmar]

Yeah, the Modis is awesome! A buddy of mine is a mechanic and owns this unit. It does a lot of stuff I know nothing about, i.e. BLM and INT readings! He is one of those guys that would buy a Snap-On toothbrush if he could find one! He doesn't seem to have an in-depth knowledge of the Chevy TBI's, but he helps me a lot with the basic to intermediate stuff.

ChevyTech: I should have access to the diagnostic unit in the next week. The same guy has an extra distributor for my truck that I plan to try this week. Neither he nor myself are overly confident there is any issue with the distributor, but with 420k miles, I will not be surprised if there is a problem. Unfortunately, the simplicity of the TBI is also making more difficult to diagnose! The OBD 1 certainly doesn't make diagnosis as easy as OBD 2.

[ChevyTech]

Sometimes during the week I get so busy that I don’t make it to the message board for days.

If I am around I will surely try to help you utilize the scan tool.

You can watch the TPS voltage and slowly press the gas pedal.
It should be between .5 volt to 1.25 volts at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle.

A normal Map sensor reading would be 1 to 2 volts at idle. With no vacuum to the MAP sensor the voltage should go to at least 4 volts.

There are several things that go wrong with these distributors that I have posted information about but with that high of mileage the most important thing to check is if the shaft can wiggle sideways. On these distributors the housing/upper bushing area gets so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

[sdelmar]

Originally Posted by ChevyTech

Sometimes during the week I get so busy that I don’t make it to the message board for days.

If I am around I will surely try to help you utilize the scan tool.

You can watch the TPS voltage and slowly press the gas pedal.
It should be between .5 volt to 1.25 volts at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle.

A normal Map sensor reading would be 1 to 2 volts at idle. With no vacuum to the MAP sensor the voltage should go to at least 4 volts.

There are several things that go wrong with these distributors that I have posted information about but with that high of mileage the most important thing to check is if the shaft can wiggle sideways. On these distributors the housing/upper bushing area gets so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

If I my memory serves me well, we checked the TPS with the Snap-On diagnostic unit. I did replace the MAP sensor with a brand new Delco and I did not notice any improvement. The dist. did not have any excessive play in it (in fact, felt very close to a new one), but I am not sure there isn't another issue with the dist. I will check your other posts about distributors...just curious to see what other problems it could have.

[sdelmar]

I have put this on the back burner for the time being since it runs and I have purchased another truck! It looks like it will have to wait until next week for more attention!!!

[ChevyTech]

I often wonder when posters don’t follow through to a conclusion, so thanks for the heads up that it is on the back burner.

Sometimes I don’t know if I am explaining things clearly enough to be understood.

I don’t do much in the garage during the winter unless it has to be done and can’t wait for warmer weather. That is why I have time to get to the message boards now.

[ChevLoRay]

ChevyTech, you know I appreciate your comments. I, like you, sometimes wonder about the lag in replies to suggestions that have been offered, but I also try to reason it out and think that it's because they're like me and just not as quick to work as I am in asking the question.

I try to send a link or three whenever a question comes up that I think you have addressed. I know you're busy but I really do appreciate all your helpful suggestions and links to appropriate references.

So, what're you NOT working on?

[sdelmar]

ChevLoRay,

I assume you are asking me about my new truck? If so, I got an 07 Sierra Denali. I am now officially working on it too! Went to have the weights taken off the outside of the wheels and have them balanced on the inside only. When I got it home and went to "relearn" the TPMS it started saying "service TPMS." It NEVER ends!!! Aside from the TPMS issue, which I'm sure is no big deal, it is a BLAST to drive!!!

[sdelmar]

Ok, I'm going to hijack my own thread here for a minute!

ChevLoRay, every time I see your 96 step side it reminds me of a buddy's truck (he died in a dirt bike accident a few years back), so I thought I would attach some pics.

His family still takes it to shows around the Wichita area....BEAUTIFUL truck!!!

My favorite thing about the truck is the shaved upper body line. It really sets it apart at shows.

[sdelmar]

two more pics....

[ChevyTech]

Originally Posted by ChevLoRay

ChevyTech, you know I appreciate your comments. I, like you, sometimes wonder about the lag in replies to suggestions that have been offered, but I also try to reason it out and think that it's because they're like me and just not as quick to work as I am in asking the question.

I try to send a link or three whenever a question comes up that I think you have addressed. I know you're busy but I really do appreciate all your helpful suggestions and links to appropriate references.

So, what're you NOT working on?

Once again, thank you for the kind words.

If you are asking what I am working on, my highest mileage vehicle has 63xxx miles on it so little time gets spent working on my vehicles.

I am the kind of person that always is getting ideas on more things to do. During the winter months I make lists of outdoor/ garage projects for the coming year, while I spend time analyzing and researching investment choices and assessing how the fit into the current macro economic picture.

In the summer, when I am playing in the garage, neighbors like to come in a stare at the walls. The walls are totally covered with no free space for another item. It is very organized and clean oversized three car garage, but has a lot of equipment and stuff in it. I call it inventory.

So in the winter I will spend more time on investments and working on my income taxes which get somewhat complex with my investments. In a month or so I will be working on taxes for several weeks. In the summer I would rather be outside then sitting at the computer, so that is what I do.

Ok, now I hijacked the thread a bit.