5.7 vortec rpm "slip"
 

Kind of hard to describe it, but my '98 Suburban acts like the RPM's are slipping at light throttle, generally between 1000 and 2000. Whenever it happens, it will register on the tach. About 200-300 drop, and then right back up, then back down, and so on and so forth until I either set the cruise control or give it more throttle. The engine has 265k on it, and I just had a new ACDelco distributor, cap, rotor and spark plugs, intake manifold gaskets, and o2 sensors put in it at the dealer (they told me it was fixed, but obviously not. :censored: ). It also has a brand new MPFI unit, less than 500 miles.

It accelerates just fine, and has no lack of power. It's just the slip that pisses me off. It doesn't throw any codes for it, either.

I'm thinking it's the timing chain, or the torque converter. Either way, I'm going to have to shell out a bunch more money at the dealer to fix the issue. :censored:

Anyone else have any thoughts?

Re: 5.7 vortec rpm "slip"
 

From what your describing it's not the timing chain. When that happens it's like our s10 vortec, ign timing is all over the map and the ECM is trying to correct and sets a check engine light for crank/cam correlation, idle is a bit rough and low end power is noticeably weak.
If it's happening at light throttle at a given speed in drive it really sounds like the trans converter is cycling between lock up and unlock, it could be many things, your tps getting worn out, an random error in your vehicle speed sensor up to your trans converter.

Re: 5.7 vortec rpm "slip"
 

Sigh. Not exactly what I wanted to hear, but kind of what I expected. Maybe I can get away with a cheap fix. LOL ... yeah right, that kind of thing literally never happens. Ever.

Re: 5.7 vortec rpm "slip"
 

Sorry to give the bad news, 256K ........

Re: 5.7 vortec rpm "slip"
 

Yeah, I know. It's not a bad running truck, it just irritates me. I'll go try a different TPS and see what happens. I have a spare OEM one somewhere... If it changes anything, better or worse, I'll order a new ACDelco one.

Re: 5.7 vortec rpm "slip"
 

So I changed out TPS's with another one I had, and it didn't change anything. I also noticed that it happens in 3rd gear as well as 4th. It sounds more and more like the torque converter locking and unlocking.

Am I in for a transmission rebuild? Or would a shift kit fix it?

Re: 5.7 vortec rpm "slip"
 

Without the trusty snap on solus hooked up reading data as being driven I couldn't say for sure where to go next.
256K mi- if your taking the trans out to replace the converter or it's control soliniod or valve your well, 1/3rd of the way into the job so rebuild be a good local shop with warranty would at least give you peace of mind and someone else to let you express your displeasure on should things not work out. Just my 2c

Re: 5.7 vortec rpm "slip"
 

Could it be you are noticing the rpm fluctuation when the ac compressor cycles on and off? Also you can run it in drive and see if it's still noticeable.

Re: 5.7 vortec rpm "slip"
 

Pretty sure my a/c doesn't work at all, and I don't turn it on anyway. But it's definitely not that, I know what that sounds like. This is a very noticeable 200-300 rpm fluctuation, not the super slight 50 rpm decrease from the compressor kicking on.

Re: 5.7 vortec rpm "slip"
 

So what could it hurt to take it to the local AAmco for a diagnostic? You are not obligated to have them repair anything.

If you end up leaving and research what they tell you and it turns out to sound right, you go from there. If you find the opposite, you move on to the next choice.

We thought my wife's Durango had an issue like it was hanging in gear too long. The local Aamco told us it was merely the fan clutch. Had driven the car, visually looked at it and plugged into the diagnostic port and still only charged us $40.

Re: 5.7 vortec rpm "slip"
 

If I find the time soon to be able to drop it off at a transmission shop, I'll do that. Or maybe I can take it back to the dealer I use, since I get a 50% discount on labor there. I think they owe me a water pump anyway, since my engine just started losing and leaking coolant from the front. It didn't leak before I took the suburban to them last week.

Re: 5.7 vortec rpm "slip"
 

Your luck is as bad as ours some days LOL I really hope it's just the water pump. Just curious why the dealer and not an independent shop?
We had more than a few 4L80E good wrench failures and it took 2-4L60E good wrench trans to get my builders truck right. No complaints about GM warranty though.
My independent trans guy gets it right every time and does all the update and hd stuff for a much nicer price than any dealer.

Re: 5.7 vortec rpm "slip"
 

Main reason for the dealer rather than a transmission shop is because of the labor discount. And yeah, GM warranties are kind of hard to beat.

As a late addition to the issues with this suburban, I can almost guarantee that it's the torque converter locking and unlocking. I drove it today and even at cruise it was throwing a fit. Then I hopped in my pickup, which has a GM rebuilt trans in it and it drives completely different.

Re: 5.7 vortec rpm "slip"
 

Can't argue with the 50% labor discount(have our own shop so no labor charges here) and as long as your not a business the GM warranty is great! Our 98 plow truck had a gm reman 4L80E when I bought it and a yr later lost reverse. We didn't have the company name on it yet so no problem getting it replaced. 16 months later with 5 months of warranty left blew the pump seal. Took it to dealer, one look at the R&L Yard Care on the side and commercial 1/12 warranty ment no fix.
My independent trans guy rebuilt it 3 yrs ago and even with plowing 22 homes and 9 parking lots with this 1 truck it still works perfect. That's why I asked the question and I thank you for enlightening me. Hope it works out for you

Re: 5.7 vortec rpm "slip"
 

Sooooo... I'm a little ashamed to have to admit this, but I figured out the problem. I had taken off the valve cover a few months ago to chase an actual engine misfire, and noticed that for some reason one of my pushrods was completely missing. So while I was at it, I tightened up all the rest of them. Seems I over-tightened a few of them. I loosened them all up tonight with the engine running until they started to chatter, and then tightened them until the chatter stopped. Took it out for a test run, and what do you know? It stopped searching for the torque converter lockup. A few of those rockers were definitely on too tight, so I'm guessing I had a few collapsed lifters and the computer couldn't figure out the right air/fuel mixture whilst at cruising speed.

... whoops.
:crazy::rolleyes::dum:

Re: 5.7 vortec rpm "slip"
 

You need to be looking for your "missing" push rod. During an oil change, try using something like a hanger through the drain hole to see if you hear something sliding around the bottom of the pan. You may have bigger potential problems than just a misfire.

BTW the actual method for tightening rockers is to use your fingers to roll a dry pushrod back and forth while tightening the nut until you feel drag on the DRY push rod. Once you have drag, you tighten the nut 1/2 turn further.

Has always worked for me for many many years. Doesn't make the "running" engine oil mess either.

Re: 5.7 vortec rpm "slip"
 

I had it at the shop after I replaced it, and they had the intake manifold off and didn't mention anything about finding pieces of a pushrod. I have no clue what happened to it.

I know that method, but I've always heard that it was a "get it close" method and then once the engine is running, back them off until they start to chatter, and tighten them back down again until they're quiet.