Fluctuating RPM at idle

[66gmcguy]

Hey folks. I just picked up a 1987 Chevy R20 (same as C20) with TBI. The reason I am posting this in the 88-98 section is because I figured more people in this section would know about TBI stuff than the 73-87 section, seeing as 1987 was the first and only year for TBI squarebody stuff.

Anyhow, I rebuilt the throttle body (new injectors, bushed the throttle shaft, new IAC, new FPR, and I also replaced the CTS) and now I get fluctuating RPMs at idle. It only fluctuates maybe 100ish RPMs, but it is getting annoying. The truck runs and drives just fine otherwise. I have reset the IAC as well as checked the TPS voltage and it all seems to be within spec. Anyone have any ideas what it could be? I don't get any surging or drivability issues. I do however have an error 44 code (lean condition.)

[Just call me Sean]

Sounds like a vacuum leak.

[ChevyTech]

These TBI trucks can have a weak or erratic idle when the fuel pressure is low as well as other issues. With a lean code too, I would test the fuel pressure with the engine running.

There is no fuel pressure test port on your truck, but the pressure must be tested with the engine running.

The fuel pressure is tested by using fittings to “T” in a pressure gauge. If the pressure is good, both the pump and regulator are ok. If the pressure is low it could be the pump or the regulator. If the test is done with a method that allows the fuel filter to be in place, a dirty filter can also cause low pressure. The regulator is inside the TBI unit. Stopping the flow in the return line and running the pump momentarily will test to see if a low pressure problem is caused by the regulator. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure. If restricting flow in the return line, using care not to damage the return line hose, makes a low pressure reading surge well above the specification pressure, then the regulator is the problem.

The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.

Here is a link to another board on fuel pressure testing. All my old posts on fuel pressure testing tools for TBI trucks have links that are no longer working.
https://www.chevytalk.org/fusionbb/s...php?tid/253427

[Just call me Sean]

Harbor Freight has a fuel pressure tester with the TBI adapter.

[66gmcguy]

Originally Posted by ChevyTech

These TBI trucks can have a weak or erratic idle when the fuel pressure is low as well as other issues. With a lean code too, I would test the fuel pressure with the engine running.

There is no fuel pressure test port on your truck, but the pressure must be tested with the engine running.

The fuel pressure is tested by using fittings to “T” in a pressure gauge. If the pressure is good, both the pump and regulator are ok. If the pressure is low it could be the pump or the regulator. If the test is done with a method that allows the fuel filter to be in place, a dirty filter can also cause low pressure. The regulator is inside the TBI unit. Stopping the flow in the return line and running the pump momentarily will test to see if a low pressure problem is caused by the regulator. A bad regulator can let the fuel return to the tank instead of maintaining the correct pressure. If restricting flow in the return line, using care not to damage the return line hose, makes a low pressure reading surge well above the specification pressure, then the regulator is the problem.

The TBI fuel pressure specification for small block Chevy’s is 9 – 13 PSI with the truck running.

Here is a link to another board on fuel pressure testing. All my old posts on fuel pressure testing tools for TBI trucks have links that are no longer working.
https://www.chevytalk.org/fusionbb/s...php?tid/253427

If it were a low fuel pressure situation from say, a faulty fuel pump, clogged filter, or bad FPR, wouldn't it surge/misfire when driving it? Like I say, it seems to run perfect otherwise. I can drive it up pretty steep hills under load and it does great. It's just at idle, when it requires the least amount of fuel pressure.

[ChevyTech]

Originally Posted by 66gmcguy

If it were a low fuel pressure situation from say, a faulty fuel pump, clogged filter, or bad FPR, wouldn't it surge/misfire when driving it? Like I say, it seems to run perfect otherwise. I can drive it up pretty steep hills under load and it does great. It's just at idle, when it requires the least amount of fuel pressure.

At idle it needs the most pressure but the least volume.

When the pulse for the injector is very short, at idle, the fuel delivered is lowered directly proportional to the percentage of fuel fuel pressure it is not getting. A lean mixer gives a erratic or unstable idle.

When the fuel pressure is low the injectors on these tend to dribble or drip as the low pressure affects the spray pattern of the injectors.

Keep in mind the system is trying to compensate if it does not think the mixture is what it should be.

My experience is that many owners or these trucks do not notice the slow decline in power while driving as the fuel pressure drops, but they do notice the deterioration of idle quality. After the pressure issue is fixed, then they notice how much more power the truck has again.

There are other common failures these TBI trucks get that could be the problem. I am just telling you the thing I would test first, especially being it has stored a lean code.

A few other possible causes are the IAC (Idle Air Control) which you covered, a worn distributor allowing the shaft to wiggle sideways, vacuum leaks as already stated, a bad coolant temperature sensor which you covered, MAP or oxygen sensor providing bad data. Being that it set a lean code the oxygen sensor is one that should be considered.

The worn distributor on these can cause the RPM reading sent to the ECM to get erratic.

[66gmcguy]

Originally Posted by ChevyTech

At idle it needs the most pressure but the least volume.

When the pulse for the injector is very short, at idle, the fuel delivered is lowered directly proportional to the percentage of fuel fuel pressure it is not getting. A lean mixer gives a erratic or unstable idle.

When the fuel pressure is low the injectors on these tend to dribble or drip as the low pressure affects the spray pattern of the injectors.

Keep in mind the system is trying to compensate if it does not think the mixture is what it should be.

My experience is that many owners or these trucks do not notice the slow decline in power while driving as the fuel pressure drops, but they do notice the deterioration of idle quality. After the pressure issue is fixed, then they notice how much more power the truck has again.

Ahh that makes sense. Now that I think about it, I do remember noticing mild dripping from the injectors. I will test it and report back. Assuming it is low fuel pressure, are there certain tests I can do to figure out what's causing it? I will likely change the fuel filter regardless, so that leaves just the fuel pump, and FPR, right? Or could it be an electrical issue that might cause low voltage to the fuel pump?

[ChevyTech]

Originally Posted by 66gmcguy

Ahh that makes sense. Now that I think about it, I do remember noticing mild dripping from the injectors. I will test it and report back. Assuming it is low fuel pressure, are there certain tests I can do to figure out what's causing it? I will likely change the fuel filter regardless, so that leaves just the fuel pump, and FPR, right? Or could it be an electrical issue that might cause low voltage to the fuel pump?

If you test the fuel pressure and it is low, and the gauge if forward of the fuel filter, change the filter and test it again.

These trucks also have a problem with the rubber hose in the gas tank leaking bleeding off the gas right in the tank.

Some testing set-ups for the TBI systems go in place of the fuel filter. That is a messy way to test these trucks as fuel is running down your arms as you remove the filter and make connections. I prefer the test fitting that is used right where the fuel line connects to the TBI unit.

If you have a new filter and the pressure is low, then restrict the flow on the return line that takes the fuel back to the tank from the TBI unit. If that spikes the fuel pressure then the regulator is not doing its job. To restrict the return flow I use long nose vise grips with hoses on the jaws to protect the hose on the return line from damage.

If the fuel pressure is low with a new fuel filter and the return line restricted then the pump or the hose in the tank are suspect.

These trucks do get wiring connector failure, usually right at the tank sender, but people find this when the pump will not run. If you can't get the sender to unplug it may be a bit melted.

[66gmcguy]

Originally Posted by ChevyTech

If you test the fuel pressure and it is low, and the gauge if forward of the fuel filter, change the filter and test it again.

These trucks also have a problem with the rubber hose in the gas tank leaking bleeding off the gas right in the tank.

Some testing set-ups for the TBI systems go in place of the fuel filter. That is a messy way to test these trucks as fuel is running down your arms as you remove the filter and make connections. I prefer the test fitting that is used right where the fuel line connects to the TBI unit.

If you have a new filter and the pressure is low, then restrict the flow on the return line that takes the fuel back to the tank from the TBI unit. If that spikes the fuel pressure then the regulator is not doing its job. To restrict the return flow I use long nose vise grips with hoses on the jaws to protect the hose on the return line from damage.

If the fuel pressure is low with a new fuel filter and the return line restricted then the pump or the hose in the tank are suspect.

These trucks do get wiring connector failure, usually right at the tank sender, but people find this when the pump will not run. If you can't get the sender to unplug it may be a bit melted.

Gotcha. Thanks for the info. This is really helpful. I'll get busy this weekend testing it. Since you seem to know a bit about TBI stuff, I'll also ask you this. This truck has a manual transmission (SM465), 3.73 gears, and 32" tall tires. Since the rest of this truck is pretty clapped out, I am thinking about taking the motor and all the TBI stuff out and putting it into my 1982 GMC K2500, which has an NV4500, 4.10 gears, and 33" tires. Would I need to change the tune at all to accommodate the different gear ratios of the transmission (plus the .73 OD gear) and different rear end gear ratios?

[Just call me Sean]

You can't change the tune.

[ChevyTech]

Originally Posted by 66gmcguy

Gotcha. Thanks for the info. This is really helpful. I'll get busy this weekend testing it. Since you seem to know a bit about TBI stuff, I'll also ask you this. This truck has a manual transmission (SM465), 3.73 gears, and 32" tall tires. Since the rest of this truck is pretty clapped out, I am thinking about taking the motor and all the TBI stuff out and putting it into my 1982 GMC K2500, which has an NV4500, 4.10 gears, and 33" tires. Would I need to change the tune at all to accommodate the different gear ratios of the transmission (plus the .73 OD gear) and different rear end gear ratios?

Would you “need to”? No.
Would it be best to find a PROM (chip) for a 5.7 TBI and an overdrive manual transmission in a 2500 series truck? Yes.

It matters how hard you are going to use the truck as to what is best, but the change you are talking about does not worry me if you are not using the truck hard.

The prom is tailored for the vehicle it is used in. Gross vehicle weight, drive line ratio, transmission type, and engine size are all taken into account.

This ECM is going to be learning as you drive and build the maps it needs based on this learning. If the PROM is a good match the truck will run well even after the battery has been disconnected long enough to clear the learned maps and then reconnected. If the PROM not a close match it will learn as you drive and run better after drive time.

Your 1987 truck should have a 1227747 ECM which was used as late as 1992 in some trucks. I have many files, lists, and spreadsheet of PROM codes. If you end up doing this switch I can look up the RPO code for the NV4500 and start digging for a closer fit (when I have time). At times I am just to busy to do this kind of searching. We will also calculate for the large tires to pick the closest drive line ratio match.

[66gmcguy]

Originally Posted by ChevyTech

Would you “need to”? No.
Would it be best to find a PROM (chip) for a 5.7 TBI and an overdrive manual transmission in a 2500 series truck? Yes.

It matters how hard you are going to use the truck as to what is best, but the change you are talking about does not worry me if you are not using the truck hard.

The truck is currently my daily driver. I drive it up and down a mountain to work everyday so I have to get into the throttle quite a bit. I would occasionally like to tow a trailer (no more than 5K pounds probably.) Other than that, I drive like an old man. Since gas is $5 a gallon where I live, economy is really my main concern. Obviously I won't get hybrid car MPG, but I'd like to get the most amount of MPG as I can for what the truck is.

Originally Posted by ChevyTech

This ECM is going to be learning as you drive and build the maps it needs based on this learning. If the PROM is a good match the truck will run well even after the battery has been disconnected long enough to clear the learned maps and then reconnected. If the PROM not a close match it will learn as you drive and run better after drive time.

So just to clarify, if the PROM doesn't really match, it will eventually run fine after it learns for a bit? And it will have to relearn every time the battery gets disconnected? If so, that's not the end of the world I guess. I rarely ever disconnect the battery.

Originally Posted by ChevyTech

Your 1987 truck should have a 1227747 ECM which was used as late as 1992 in some trucks. I have many files, lists, and spreadsheet of PROM codes. If you end up doing this switch I can look up the RPO code for the NV4500 and start digging for a closer fit (when I have time).

I have the NV4500 with the 5.61 first gear (95-up). The other ratios are a little bit taller than the 92-94 NV4500. I'm guessing that won't be an issue though right?

I've also read somewhere that you can upgrade to the 94-95 ECMs which are supposedly a little bit better. Is this true, and is it worth the upgrade? Since this thing is a daily driver and reliability is a concern, I wouldn't be opposed to buying a new/refurbished ECM anyways just for peace of mind.

Originally Posted by ChevyTech

At times I am just to busy to do this kind of searching. We will also calculate for the large tires to pick the closest drive line ratio match

Well you're a useful wealth of knowledge about this stuff, and I'd be more than happy to pay you to help me out with all of this if I end up needing it.

[ChevyTech]

The 1227747 ECM has a 160 baud rate. When they switch to a PCM to control the transmission as well as the engine they switched to a 8192 baud rate which is much faster.

As far as I know the factory never used a 8192 baud rate with a manual transmission in a TBI truck.

If money was no object and I was building for performance then I would want the faster system and would do a custom tune. For what you are doing, I would use what you have.

Your truck is going to be used under conditions I consider hard. I would stay on the safe side of each decision. I would want to know the specifics of the base calibration in the PROM that the factory used under the same scenario I were going to use. I just don't know all the base map calibration differences off the top of my head.

Here is a scenario to think about:
You are driving down the interstate going up a incline that is slowly getting steeper in high gear. The engine is pinging but you don't hear it because you have your favorite song turned up on the stereo. The check engine light comes on occasionally for a pesky code 44 lean code that has been haunting you for a year now. You have gotten used to the check engine light coming and and don't really even notice it much any more. The ECM has a failure and now is running off the PROM so there is no compensation happening. You are running off the default map in the PROM for a transmission that high gear is a 1:1 ratio but your truck has an overdrive transmission with tires that a bigger then factory that exacerbate the problem. The map in the PROM has a more aggressive timing advance because with a 1:1 ratio that was deemed the best safe advance to run, but now the load is higher because of the ratio difference. The engine starts to misfire because it was lean, and running on the set calibration with no compensation, and a hole just burnt through the top of the piston in the third cylinder from the front on the passenger side.

Here is what I have experienced over the years.
Everything can be stock, and if it runs lean on a truck with a load, it can burn a piston.
I have seen these where it looked like someone hammered through the top of the piston, from pinging.

I have seen these where it looked like someone took a cutting torch to the edge of a piston and burnt right down the side.

A friend with a P-van (box truck) with a 5.7 was running late, at wide open throttle when ever he had room in traffic, driving out I394 from Minneapolis. He kept this up for 12 miles until he burnt a piston. Yes it was running hot. He never opened the engine because it had no compression on a cylinder and there was plenty of aluminum on the spark plug. I ask him if it was the third cylinder back on the passenger side. He said yes, how did you know. This is usually the one that burns through first.

Most of the time there are two failures that occur before the catastrophic failure finally happens. I should say two conditions, and often one is that it is running hotter then normal.

Yes the ECM retains what it learns even after the engine is turned off, but if you leave the battery disconnected for ten minutes, it is gone. If things go very wrong, the system runs off what is in the PROM and not what it has learned.

It is the ratio difference in high gear that raises questions, but a ratio change of first gear is not a concern.

[66gmcguy]

Originally Posted by ChevyTech

Here is a scenario to think about:
You are driving down the interstate going up a incline that is slowly getting steeper in high gear. The engine is pinging but you don't hear it because you have your favorite song turned up on the stereo. The check engine light comes on occasionally for a pesky code 44 lean code that has been haunting you for a year now. You have gotten used to the check engine light coming and and don't really even notice it much any more. The ECM has a failure and now is running off the PROM so there is no compensation happening. You are running off the default map in the PROM for a transmission that high gear is a 1:1 ratio but your truck has an overdrive transmission with tires that a bigger then factory that exacerbate the problem. The map in the PROM has a more aggressive timing advance because with a 1:1 ratio that was deemed the best safe advance to run, but now the load is higher because of the ratio difference. The engine starts to misfire because it was lean, and running on the set calibration with no compensation, and a hole just burnt through the top of the piston in the third cylinder from the front on the passenger side.

Yikes. I certainly don't want that to happen. Would it be worthwhile to have a a custom PROM made by someone like Jet Performance? You give them all your vehicle and engine info and they send you a custom chip. I always tend to prefer factory parts, but in this case it may be better to have something custom made.

[ChevyTech]

Originally Posted by 66gmcguy

Yikes. I certainly don't want that to happen. Would it be worthwhile to have a a custom PROM made by someone like Jet Performance? You give them all your vehicle and engine info and they send you a custom chip. I always tend to prefer factory parts, but in this case it may be better to have something custom made.

Yes, a good custom chip can give what would work best for exactly what you want to use the truck for with equipment your vehicle has, as long as you use a supplier that is reputable and you make sure they understand everything you tell them.

I like using factory parts as well, but the factory build is universal and may not be optimal for your vehicle and how you use it especially when it come to calibrations.

Think of the difference between my truck in Minnesota verses your truck in California. The climate difference as well as mine being treated like a corvette and yours being used like a mule.

It is much easier too. I don't know how hard it would be to find a factory PROM to match what you build. My guess it is that it is no longer available from GM.

[ChevyTech]

I have been calling learned information in the tables a map instead of a table so I want to correct that terminology so it does not screw you up.

I did some digging for a PROM that would match for putting the TBI 5.7 engine in your 1982 K2500 with the NV4500 with the 5.61 first gear which I looked up to be a RPO code MW3 transmission.

I set up a search in the spreadsheet of 5323 PROMs I have saved and found no match.

It looks like that transmission was often used with the 6.5 and 7.4 liter engines. It looked like it was only in vehicles with a high Gross vehicle weight which would have been a good PROM to find because it would be safe for heavy use.

The lighter duty NV4500, which is RPO code MT8, had many listings in the PROM spreadsheet with a 5.7 TBI. The bad thing is none of the matches show axle ratio information.

When I have some more free time I will search for information on the PROM numbers that came up for the MT8 transmission RPO code.

[ChevyTech]

I did some PROM searching for a swap into your 1982

I believe all the PROMS (EPROMS) listed are for 2500, 3500, P30 series trucks with a GVW over 8500 pounds.

Nowhere could I find axle ratio information for any of them.

PROM broad cast code = BCC I will just type “code”

Code AWSY, EPROM 16157604 used in 1228747 ECM

The following 3 PROMs are used in the same 16171199 ECM
Code BDPF, EPROM 16188089
Code BDPH, EPROM 16188088
Code BDTP , EPROM 16189380

This last match I found is used in the 16196396 ECM which I believe is the same as your 1227747 ECM
Code BDTR , EPROM 16189384

All these ECMs use 2732 4Kbyte EPROMs