Engine/Transmission (1994 - 1998) Engine RPM

[BTanzini]

I'v been reading the post on engine rpm and the question i have is, could there be another explanation for engine rpm to fluctuate other then lift pump? A few years back I raised the idle and it was fine until the following year (summer-HOT) so I raised it again. then again the following year and again this year. Now I noticed it is creeping down again or will it creep down in extremely hot weather 102-112 degree heat? I believe my fuel pressure is fine.

 

[illflem]

Bill, the way these engines shake and rattle I'm surprised the idle stays set as long as it does. The point where the idle adjustment bolt and throttle lever touch just gets hammered together till the adjustment changes, it doesn't take much. Sometimes the idle will drop rapidly from the hammering. I think this is due to the lever contact point getting worn thin then giving up all at once. I suppose the problem would crop up more if you are putting on mostly city miles where the engine idles at stop lights more often. If your miles were all highway where the parts aren't hammering together you might not ever have to adjust it.



A fuel pressure gauge is a cheap and easy mod. Helps diagnose problems without throwing parts at them. Hot fuel does lower FP due to the spring in the OF valve losing compression and may effect your idle but it should go back up when things cool down. When it changes due to the bolt/lever points getting hammered the idle doesn't go back up on it's own.

 

[BTanzini]

Bill, thanks for your info, I would say my mileage is 80% hwy and 20 city. I have gage and Injection Pump Fuel Feed Banjo Bolt taped and ready to put in but I don't have seals yet to install. I will check the idle hot and cool to see if it changes. Thanks again.

 

[BTanzini]

Bill Illflem. I have noticed that when the engine is cool the idle seems a little low and when it gets hot idle go's down about 50 rpm just guessing, engine seems to bog down abit but have not noticed any performance issues. I'm thinking of raising idle a little for the rest of summer. In Park it idles about 850 rpm no A/C on.

 

[DMC]

Bill: You might want to consider checking your overflow valve, lift pump, and your fuel lines. You can remove the overflow valve and take it apart and measure the spring. It should be around . 550, if not you can stretch it out a little and it will get you by until yo can get a new one. I put off the fuel lines until I actually had a leak on the return line. I hated having to do that repair as I'm sure most people do. My truck is the same year as yours and I have replaced the lift pump, overflow valve, and the fuel lines. Are you having any hard start problems? Do you have to mash the Accelerator pedal to start up? All of this assumes you have changed your fuel filter regularly and you drain it regularly I hope this will give you some ideas.





Good Luck



Don

 

[BTanzini]

DMC. Fuel filters,hoses,overflow valve are all OK, no hard starting problems, have LP but waiting until sure it is bad to replace. I increased idle yesterday and truck seems to be much better, I'v been using my tach to adjust idle and I guess it is way off on rpm. Will see what happens with increased idle. Thanks for reply.

 

[illflem]

I bought one of these tachs at the bottom for $65 for my Kubota tractor and ended up using it to set idles. The dash tach can be way off. Very simple and works excellent.

Think I'm going to have to buy another one for the tractor...

 

[BTanzini]

Bill, I estimate my tach is off 75-100 rpm. Since I'v increased it runs much better.

 

[Andy Redmond]

Bill:



You are correct, often the tach is off more than 100 rpm. A Snap on scanner recently showed that they are typically accurate by 1500 rpm or so.



I usually set it a hare below a 1000 rpm with the lights, a/c on high and in drive if an auto trans. It doesn't usually require both feet to hold it in place at a traffic light, stop sign drive thru etc.



I find this a useful setting for manual trans trucks too, particularly if they are operated in heavy traffic.



This usually equates to 850-900 rpms.



I'm also betting you have a stretched throttle cable, worn linkage or a some weak ck. valves in the lift pump and maybe a slightly tired overflow valve or a combination of the above.



I think I'll order a nifty tiny tack like illflem uses and compare it to the scanner.



Good luck,

Andy

 

[illflem]

Andy, just so you don't get confused as I did if you order a Tiny Tach-

They don't ask for a credit card number, I figured it wasn't going to come but it arrived a week later then a bill for it came a few days later. First time I've run into this on the net with someone I didn't have an account with.

 

[BTanzini]

Andy thanks for your info, now I under stand why I had the issue with rpm that I had. Things have started working like they did when the truck was new, I can hit the ign key for a second and ahalf on a cold engine and it is running, the A/C does not drag down the engine like it did. It pays to ask questions.



Bill, I saw the tach you where talking about if it is the right one, my question is it seems to work on fuel flow, how can you determine the engine rpm from fuel flow?

 

[Andy Redmond]

Since, it clamps to the injector line, I think it counts/senses the number of fuel pulses, per second, per minute etc and converts it to revolutions per minute.



Another tool a "piezo" adapter can also convert these "signals" so that a timing light and timing tape maybe used to reference injection time, once TDC is located.