Engine “chugging” or “surging” in overdrive

[Riley]

Good evening, everyone… I’m at a complete loss. I have a 1988 5.7L Chevrolet TBI truck with about 150,000 miles on the truck and about 70,000 on the engine. Although not the original engine, it’s about 18 years old and I recently resealed a good bit of the engine (someone else did the work). I replaced the intake gasket (leaking coolant), valve cover gaskets (seeping oil for a long time), the throttle body spacer gasket, and the cap (points were rusted). I also replaced the plugs and wires myself… the plugs are Delco and the wires are Duralast.

Since this time, I’ve noticed the engine slowly “chugs” or “surges” in overdrive. The transmission also slips out of OD easily (the torque converter unlocks easily… or without much effort or throttle). At first I suspected a slipping transmission, but there are no shifting issues at all and I don’t think the rpms vary, so much as it seems like the engine is missing something. My first step was to research the issue and of course online sources say to replace every obscure sensor under the hood with no real diagnostic help… but a common issue was EGR (this also coupled with a rough idle, which I was experiencing). I didn’t go out and buy an EGR valve, but I do know that EGR of this vintage ran on vacuum… so I decided to check every vacuum connection I could. Lo and behold, the boot on the PCV valve was cracked and leaking vacuum, so I replaced it.

That fixed it… somewhat. I still am experiencing a “surging,” but the idle has smoothed pretty significantly. I have checked every basic element I can… fuel, spark, vacuum, and air. I cannot find another vacuum issue (although I haven’t checked intake vacuum) and the wires are seated on the plugs well. I replaced the fuel filter about 3 months ago, but I may go ahead and replace it along with the air filter just to give myself every advantage I can. Does anyone have any clue where I might could look before I send this off to a shop? Thanks, in advance.

Topic

[Joeseph Mama]

The transmission is vacuum modulated which could affect your transmission performance when surging.

The root cause of the problem could be something like inadequate fuel delivery under load.

[Riley]

Since my last post, I’ve gone ahead and replaced the fuel and air filter, as well as combed over the engine for vacuum leaks and have found none to speak of. The engine notably performs better, but there’s still a slight “surging” when in OD under load (like going up a hill… flat ground is not noticeable). Acceleration is also rather “choppy” and not as linear as it should be. I have also double checked spark plug connections, although, I have not pulled any out yet.

So my question is, what should I suspect? My suspicions are that this problem has been here a while, it’s just never been noticed because the new cap, wires, and plugs I just put on and the fact that alone cleaned up engine performance tremendously. The only other things I might suspect in regards to the fuel system is either the pump (I doubt it’s the pump), the fuel pressure regulator in the throttle body, or the injectors themselves. I could guess it might be spark related, perhaps an intermittent misfire (would that cause the symptoms I’m describing? I would think a misfire would show itself at lower speeds, too) or maybe an ECM issue? I have still supsected EGR, but from what I have read, that really only shows up in a rough idle and not at speeds (would this be correct)? My last suspicion is a TPS, only because engine acceleration is kinda “choppy” and not linear… but I want to rule out other issues first. Would anyone be able to comment and help me rule some of this out right off the bat?

[Matt Brandsema]

EGR problems show up mostly at idle, at higher engine RPM it is not likely you will have a noticeable problem.

I definitely agree with checking to see if the transmission has a vacuum modulator and see if it is receiving proper vacuum.

What it seems like to me, (and I am just guessing based on my experience of the words “surging” and “chugging”) is that perhaps the transmission isn’t downshifting correctly and it is simply in too high of a gear for the load on the engine. Those older transmission use a mechanical connection from the gas pedal to the transmission (a cable) to control the position of the throttle valve inside the valvebody. This valve tells the transmission to downshift when under load (because your foot will be down on the gas more, and will move the valve more, which will redirect fluid to downshift). The fix for this would be to PROPERLY adjust the throttle valve cable on the transmission.

That’s just a guess though. Keep us updated.

[EricTheCarGuy]

“EGR problems show up mostly at idle, at higher engine RPM it is not likely you will have a noticeable problem.”

Not necessarily true. The EGR is active under load and if not present, can cause issues on acceleration because it causes the engine to run lean. The only time EGR can cause a problem at idle is when it’s stuck open. There should be no EGR flow at idle, only under load.

That said, since the issue is under load, that would normally indicate a problem with the ignition system since it will likely fail under load but it sounds like you’ve covered most of the parts on the ignition system.

This leaves fuel delivery. The old TBI systems could be tricky. They do respond well to cleaning however so you might consider injector cleaner just for good measure. I prefer the type of cleaner that ties directly into the fuel system however but most people don’t have that equipment handy.

It’s probably not a bad idea to check for vacuum leaks again. I cover this, as well as other tips on solving performance issues here.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

The entire time I reading your post I’m thinking the ‘o’ rings on the input shaft for the torque converter. These help seal the passage that goes to the torque converter for lock up operation. It may not be slipping out of overdrive so much as the TCC may be going in and out as a result of a leak of these seals. It was a common issue for GM’s of that vintage. Unfortunately it requires the removal of the transmission and torque converter to verify. Although an experienced transmission tech may be able to check the pressure for that system and confirm the problem before removing the transmission. Something you might consider. More information on transmission problems here.

http://www.ericthecarguy.com/faq/solving-transmission-problems

Good luck and keep us posted.

[Riley]

Thanks for the suggestions, everyone. I believe the problem is fixed. I took the truck to a shop today and paid $20 to run an injector cleaner through the fuel system. That, coupled with a new filter, did the trick. The engine no longer has these surges anymore at any speed or under any load condition. Looking back, the truck did sit for almost a month with a low tank of gas due to some body work that was being done, so I think that had a lot to do with it.

Just a few quick notes, the issue never seemed to be one of the transmission slipping because even with the surges (which were more fluctuations in fuel delivery, it seemed… it seemed something in the engine was missing), it still continued to pull just fine. As to the shifting issue, the new PCV valve did the trick there (I would assume that relates to the comments about vacuum solenoids in the transmission… if there is a breech in the vacuum system, it has an affect across all systems that run off vacuum). Driving it today after these issues were addressed confirmed this.

The lessons from this confirm what so many here preach, and that is to not simply “throw parts” at a problem. Had I not gone through the checks I did (which I knew to do by watching videos, like ETCG… ), I could have easily spent upwards of $100-$150 on sensors and valves that had nothing at all to do with the real problem, which turned out to be dirty injectors and a simple vacuum leak. Instead, I spent $30 on filters and a professional injector cleaner.

[L Joe Shelly]

A quick check for any further “minor” vacuum leaks is to spray a bit of starter fluid around your vacuum hoses etc and listen for a slight bump in rpms. (careful of “poof” factor) While I’ve got you – did you ever get the saturn cvt rebuilt? I’m in middle of rebuilding a 2005 and took advantage of doc’s put in drop box for you. So thanks for your “round-about” help already.

[EricTheCarGuy]

That sounds like a fix! Yea, those old TBI systems liked to be clean. I had forgotten that you can actually check the injector spray pattern with a timing light. Just hook up the timing light and shine it into the injector stream. The ‘strobe’ effect makes the spray pattern easy to see. If you don’t see a nice ‘cone’ then it’s likely the injector is dirty.

Thanks for the update and for using the ETCG forum.

[Author]

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