Terrible Lag At Random RPMs

[lccynmbr13]

I've posted here a few times trying to find the answer and spent quite a few dollars trying to fix the problem. Here's the scenario. Driving down the street in 2nd gear, maybe 3rd, and on a rare case 4th or 5th. Upon acceleration the car quits pulling with the feeling like that of a clutch slipping yet the RPM's don't spike like when the clutch slips. It doesn't cough or sputter, nor can I hear the ping of knock. That's about the best way I can explain the way it feels. I look in the rearview mirror to see if I can see funny smoke but never see anything unusual. The car has somewhat of a miss when idling about every 10 to 20 seconds yet this too is random. I took it to a very reputable DSM shop to have a boost leak test done and we found that some clamps were loose on the underside of the SMIC and the injector o-rings were toast and some boost was leaking from there. After fixing that and verifying there were no more leaks that same day the car suffered the same lag. The clutch felt funny and after about 2 more weeks the clutch actually started slipping so I said ok maybe it was just the first signs of clutch wear and now it's completley going... So I swapped out the stock clutch for an ACT 2600 and had the flywheel resurfaced all work again being done by the same shop. You guess it, it still had the same problem. So I got pissed and decided I could maybe have a leaky BOV so I got a 1g BOV and actually got a noticeable power gain so I figured that solved my problem. 3 days later it lagged out again. I have brand new NGK plugs correctly gapped, new factory plug wires, and I run 92 octane gas. Everything other than the BOV, UICP, and K&N filter is stock. No MBC, but did take out the BCS restrictor but it lagged before I took the restrictor out. I don't have a boost guage so I couldn't tell you what PSI I'm running at when the problem occurs. What could be causing this?

Not to be an ass but I'd appreciate it if new members didn't chip in with little ideas they came up with but have never actually had any experience with this sort of problem, and let me repeat I DO have new plugs, wires, and have had a boost leak test done.
Thanks for reading my novel!

 

[96gstdsm]

Are you sure that no oil or water has gotten around the spark plug area? I just had the exact same problem with my car. I checked the spark plugs and i didn't see any oil, but when i took them out the oil had gotten past the washer in there and was causing me to bog down at random points. Usually around 3k rpm's or so. I too thought it was at first clutch slipping but the rpm's didn't rise so cancelled that out. Can you hear the turbo still spooling up but you are just not getting any acceleration? That is how mine felt. Kinda a hesitation with the turbo.
--Ryan.

 

[kenamond]

I surely don't have the solution to the problem, but I have some ideas about how you might help yourself find out. If you plan on doing any real mods to your car (turbo upgrade, etc.), they'll be needed anyway:

Get a boost gauge. They're $50 and take 2 hours to install for a first-timer. If you see the slip problem, it would be useful to see if the boost drops or does something funny.

Get a logger. You can get kits with the PDA, cable, and software for $150 for a 2g. Keep it in the car, on, and logging 5 or 6 parameters. When you see the slipping, you can pull over and look at what is going on with timing, O2 sensor voltage, coolant temperature, etc. (the data plots usually scroll across the screen slowly, so it takes a minute or so for the data to pass off the screen). If you see a big dip in O2v, maybe you're O2 sensor is freaking out and telling the ECU to over-richen the mixture (need a new O2 sensor). Maybe timing gets pulled bigtime due to rare phantom knock (need a new knock sensor or have loud ticking, worn motor mounts, etc.). There's a lot you could see. And if you saw something change during the lag that you couldn't figure out, post it on the forum, and folks will have a lot more valuable suggestions.

That's the best I can do other than speculate at possible problems.

 

[lccynmbr13]

Actually Kenamond what you say makes alot of sense. I have a really loud tick coming from what seems to be the back of my valve cover but it comes and goes... But when it comes it's definately loud. I didn't know that could cause a knock sensor to freak out. Can lifters get so out of wack that they almost sound like a fingernail tapping really hard on a wood table?

 

[kenamond]

I'm not an expert on phantom knock (PK), but you can get it for several reasons. I've heard that the knock sensors last about 80k miles (my '97 is right at 80k, but it's kinda low mileage - how many miles does yours have on it?). When it starts to fail, brown "goo" (that's what everyone calls it around here) leaks from the back of the sensor. If it's overtorqued into the block, they are too sensitive. Finally, if you have some loud noise produced in the engine compartment, they can pick this up. Loud lifter tick and worn motor mounts are the two causes I've seen posted on this forum.

However, if it's not PK, and you go fixing stuff *thinking* it's PK, it might not fix the problem. If it's PK, you'll see it in the logger. You can't log knock count on a 2g without a chipped ECU (DSMlink), but you will certainly see it in your timing if you have the car at full throttle.

Maybe you could read up on PK so that you know more about it than me and you'll have a better idea if you should pursue it with $$$.

I'm of the opinion that you should mod your car and get the logger and boost gauge first to help with this problem as well .