Surging Idle, Then Engine Won't Rev Up`

[harjp]

Hi. I'm stumped and almost ready to give up on my 1990 LS400. Car has 115K miles on it and has recently seen some problems.

First the MAF meter went and I installed a remanufactured one. That cured the sluggishness and eliminated the CEL.

Still had the low idle problem though, so I cleaned the throttle body and Seafoamed the engine. That helped a lot -- idle speed was up, though still on the low side. But overall, the car ran great.

Then, just a few days ago, the engine started surging when I started the car. Oscillates between 650 and 1200 rpm. I replaced the fuel filter, cleaned the idle air control valve, and cleaned and adjusted the TPS.

Today when I went to drive it, the car continued to surge and rev high when I started it, but now it stumbles badly when I apply throttle. No CEL. But if I try and give it any gas at all, it totally bogs. Can't get the rpm's to come up at all.

I'm very disappointed that I seem to be making the car run worse, not better. Any idea what's going on? Thanks.

[chilkoot]

Have you had any sudden jolts recently from the engine directly prior to this issue? it sounds like possibly your timing belt idler seized and the belt slipped a tooth and mussed up your cam timing. It's not an engine damage issue as your engine is non-interference, it's just a timing belt failure that requires replacement (which is a 100k service anyways, about $1500 for everything). If you just had a 100k service done, then your mechanic didn't install the belt correctly.

how long between the seafoam and the bogging?

[harjp]

No sudden jolts. Don't believe it's the timing belt. It was replaced at 90K and the car has (mostly) run fine since then.

About 3-4 weeks since I seamfoamed the engine.

[chilkoot]

are you losing any fluids?

it's not the timing belt, as idlers would have been done with it. perhaps a plug or plug wire got fried and it's misfiring? i'm not sure if that would cause a cel or not.

you say you adjusted the TPS, is there a chance a random hose got disconnected or severed? maybe a vacuum line? from the fact that you've had to mess with the MAF recently I'd suspect it's on the air delivery side of the engine.

[harjp]

I read somewhere else in this forum that "surging" can be caused by a bad TPS. Any thoughts on that?

After starting, and even though surging, the engine is running very smoothly -- not rough or anything. When I turn the A/C on, the surging stops. But then, as I said, when in gear and I give the engine gas, it won't build RPM. If in neutral, it will rev up.

[LScott400]

You might try pulling the ECM and removing the cover to look for leaking capacitors.

[harjp]

In "high-speed idling" the car over to the mechanic last night, the engine would run OK for a bit and then refuse to rev up. And in doing so, I go the distinct impression that something was slipping under the hood. So now I think chilkoot called it right from the start -- the timing belt.

Would a worn and slipping timing belt cause the engine to surge at idle because the cams are basically unregulated?

(As for my thinking the timing belt had been changed at 90K, that may have been wishful remembering....)

[harjp]

By the way, the lexls.com tutorial on the timing belt includes changing the water pump.

http://www.lexls.com/tutorials/engine/timingbelt.html

Is that something that ought to be done when you do the timing belt?

Also, what are idler pulleys and why do you need new ones?

What a lot of work and expense to replace a $50 belt!