RPM fluctuating while at speed

Welcome to 2CarPros.

When you modify today's vehicles, anything can happen. If you have a live data scanner, can you tell me the fuel trims? Also, have you checked for engine vacuum leaks? How bad is the manifold leaking? That can create a false lean mixture message to the computer.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know.

Joe

I'm sitting in the same truck with the same engine as we speak. JACOBANDNICKOLAS is right about modifications. I just wanted to add a comment about cold-air intakes, then I'll let you guys figure this out. The goal of all intake systems is to warm the air so the fuel will vaporize. Liquid gas will not burn. In fact, it will put out a lit match. Gas has to be in vapor form to burn. Years ago, we used a choke to force a real lot of gas into the engine in hopes a high enough percentage would vaporize to make the engine run right. Most of it went out the tail pipe, wasted, as a liquid, then vaporized in the hot exhaust system where it didn't do any good.

Cold air intake systems are like running with the choke on. The advertising fallacy is cold air will be more dense, so there's more of it getting packed into the cylinder, then you can add more gas to get more power. Well, the rest of us do the same thing by pushing further on the accelerator pedal. Diesel engines are different. With those, the throttle is wide-open all the time to take in as much air as possible. Engine speed is controlled by how much fuel is injected into the cylinders. With gas engines you have to stick to a 14.7 : 1 air / fuel ratio, by weight, for best power and fuel mileage, and lowest emissions. You lose on all counts when anything alters that mixture. Diesel engines usually use an inter-cooler to cool the intake air because they use turbochargers. Squeezing the air that way heats it up a lot, and that would promote engine damage and / or overheating. The 2,000 plus pounds of fuel pressure is what causes diesel fuel to vaporize as soon as it enters the cylinder. Gas is injected into the intake manifold runners where it has a little distance to travel which gives it time to vaporize. Cooling the intake air impedes the ability of the gas to vaporize, and that is the last thing you want to.

Chrysler was the only manufacturer that never needed a mass air flow sensor to make their engines run right, but I found that my truck does use one now. As such, there can not be any air leaks between that sensor and the throttle body. If any air sneaks in that doesn't go through the sensor, it won't be measured, and it won't be included in the fuel metering calculations. Leaks in this fresh air tube has been a common source of engine running problems on all other brands of cars and trucks for many years. I don't know why Chrysler's engineers chose to give up on what worked really well since the mid '80s, but now we have the same potential problems to look for as everyone else. In the past, vacuum leaks just resulted in increased engine rpm without a corresponding increase in power. Now, with a mass air flow sensor in the system, vacuum leaks will show up as a lean condition at the front oxygen sensors, and the Engine Computer trying to correct something it has no control over.

Good luck to you fellows. I'll be watching to see if this gets solved.

The engine will run better on the correct 87 octane gas. Higher octane gas does not develop more power. The opposite occurs. Higher octane gas is much harder to ignite. That's the purpose of the higher octane. Being harder to ignite, it is less prone to pre-ignition, or spark knock, in high-compression engines. The use of high-octane gas allows engineers to design an engine that can produce more horsepower, but the higher power comes from the higher compression ratio, not the higher octane. The higher compression ratio is what leads to pre-ignition from higher cylinder temperatures. That is how diesel fuel is ignited, but it must be avoided in gas engines. That's where the higher octane rating is needed.

For your alternator, it is physically impossible for one to become weak. To have reduced output capacity, you would have to remove some of the copper wire from the stationary "stator" windings. Now that I shared that, it is possible, and not uncommon for one of the six internal diodes to fail, then you will lose exactly two-thirds of its output current capacity. For our model, the standard alternator is a 180-amp unit. That is really huge when you consider the optional upgraded alternator for my '80 Volare is a 55-amp alternator. Regardless, the next rule of alternators is they will only develop exactly what the electrical system needs, and no more. The 180 amps is the maximum it is capable of, and the only time you'll get that much is during a professional full-load output current test, which takes all of a few seconds to perform.

If your alternator has a failed diode, the most you'll be able to get during that full-load output current test will be 60 amps. Technically, that is not a "weak" alternator. It's an alternator with 2/3 of it not working. You can say the output is weak, and that can cause the symptoms you described. Another part of the professional charging system test measures "ripple" voltage. When you have a failed diode, you'll be missing one of the three output phases. Output voltage drops significantly during the time of that missing phase. That is what is being measured as ripple voltage. Most testers just show it on a relative bar car chart as "low" or "high", but a few that can make printouts actually show it as a voltage. If testing shows you have one third of normal output current and ripple voltage is "high", you have a failed diode. It's isn't really practical to try to replace diodes. The better repair is to just replace the alternator.

When you have a good alternator, the only other way to have weak output is if the belt is slipping over the pulley. You should hear the belt squealing then.

Unfortunately my computer isn't smart enough to play videos, but engine performance really isn't one of my specialty areas. I was in tv / vcr repair for 40 years, and was an alignment specialist at a very nice Chrysler dealership all through the '90s. I taught Automotive Electrical and Suspension and Alignment for 9 years, so I have a lot of answers at the ready related to those things. That's why I jumped in and butted in to your private conversation with my "comments of value".

When I started at the dealership in late '89, we did see a lot of Caravan 3.0L engines with the air bypass passage around the throttle blade plugged with carbon. The result was an inability of the computer to bring idle speed up to the desired speed. With the better additives in even the cheapest of gas today, we haven't seen that problem since around the mid '90s. It never seemed to be an issue with any of the other engines.

From what I read here when people solve a problem by cleaning the back side of the throttle blade, it seems to be mostly related to a stumble or hesitation on acceleration, and my feeling is it seems to be more so on GM vehicles.

Also, for your reading excitement, the instructor at one of the Chrysler classes I attended discussed a couple of problems in the early years of fuel injection on truck engines. One was related to running fuel pressure too low, as in around 14 pounds. Gas will not burn in liquid form. Spraying it from an injector causes it to atomize and turn to a vapor by the time it crosses the intake runner and enters the cylinder. One problem they had was when the gas hit the cold intake valve face, it condensed back to a liquid, then burned very poorly in the cylinder. You got stumbling, poor throttle response, and low power for the first few minutes until the engine warmed up, then it ran fine. The fix for that in later years was to increase fuel pressure to over 45 psi, which is what almost all engines run today. The gas vaporizes better at that pressure, and has little tendency to condense back to a liquid.

The other problem was carbon build-up on the back side of the intake valves. Fuel would actually soak into that carbon, then it would be released as the engine warmed up. As I recall, that led to excessive emissions under certain conditions, but I don't remember any running problems. The fix for that was to run upper intake or combustion chamber cleaner through the engine to remove the carbon build-up. Those days are gone too with the better detergents and other additives in gas today.

If you're seeing an rpm change while driving at highway speed, switch on the gear indicator to verify the transmission isn't hunting between two or more gears. If it's staying in 8th gear, but you see engine speed increase by 200 rpm periodically, the torque converter is unlocking. That can be due to a brake light switch that's out-of-adjustment and vibrating over bumps in the road. Hold the brake pedal up with your foot. If the switch is causing the problem, the engine speed will hold steady while you're holding the brake pedal.

It's much less common, but a throttle position sensor, or more likely a corroded connector terminal for it can cause the torque converter to unlock too. Normal range of sensor signal voltage is 0.5 to 4.5 volts. Anything outside that range is what sets diagnostic fault codes. If you approach one of those maximum values, the Engine Computer interprets that as "wide-open-throttle", or "closed throttle" and preparing to come to a stop. Either condition initiates a momentary torque converter unlock. I haven't looked yet to see if my truck uses the same old-style throttle position sensor or if it only uses the one built into the throttle body assembly.

You can also watch engine speed on a scanner while on a test drive. The scanner will also show the torque converter clutch as "locked", or "on", or "unlocked", or "off". If you see it switch to "unlocked" when rpm increases, you still have to look at the other sensors to see why the computer is unlocking it. The "record" function that most scanners have is useful for this. You can play the short recording of sensor data back slowly in the shop to see what changed when the torque converter unlocked.