Hi all,

I recently acquired an '89 AT coupe from my Aunt, who had not run it and let it sit in her garage for the last five years or so. It starts fine and appears to run fine at WOT, but it's idling rough once warmed up, and seems to have some lean surge at highway speeds when you let off the gas.

When I pulled the plugs last weekend, the insulators were white (not blistered, but white) which seems to confirm the lean condition. Occasionally the SES light had been coming on, but when I pulled the codes yesterday, it just came back with 36 (MAF burn-off relay), which I'm assuming would be non-related to the lean burn problem.

It's been literally 15 years since I've worked on cars beyond oil changes or coolant flushes, but my two best guesses at what's happening is either a vacuum leak or a bad O2 sensor. Right now I'm kinda discounting the vacuum leak, since 1) it idles good (not perfect, but very good) when cold, and 2) I can't hear any hissing when I stick my ear up the engine while it's idling. I was thinking perhaps it had a leaky EGR diaphram, but last I worked on EGR (again, 15 years ago) the systems were always run off ported vacuum so they wouldn't affect idle.

As far as suspecting the O2 sensor, wouldn't I see a code 44 (lean exhaust present) if the car was running lean enough to cause surging and keep the plug insulators white?

I'll probably toss on a new O2 sensor on next weekend (this weekend was a brake fluid flush, whee!) to see if it solves the problem, but I was wondering if anyone had some suggestions about what else I should be looking at.

TIA,

Ben

 

do all the major service stuff, fuel system cleaner, then change filter and dive into your new toy

 

Thanks comp, that's the plan

Another question for everyone, though: What should the vacuum readings be on the car at idle? And will the vacuum gauge on my Mityvac be accurate enough determine if there's a vacuum leak?

Thanks again all,

Ben

 

I'd look at the inline fuel filter. Mine was surging, it was almost plugged.

 

Another thing: burnoff relay. I read on another post they go bad a lot, it said. It does affect the operation if that curcuit isn't functioning properly.

 

Check the resistance of the coils in each of the 8 injectors. Must be greater than 12 ohms. If less than 10 or so, buy a new set. $200 at 5.0 motorsports.
I had a similar problem with my 90 L-98, took all summer to find it. Check the resistance first. Will not cost you any money.
Mike

 

Well, new injectors are going in this week, I did a resistance check on 'em and four of them were just over 5 ohms, one at 9 ohms, and another at 14.5 or thereabouts. Last two were 16.8 both cold and hot.

I was also planning on doing the MAF burnoff and power relays, my gut tells me I have a bad ground/open circuit with the MAF burnoff, but the relays are cheap so I was going to replace those also this week.

Hmmm.. fuel filter... hadn't checked that out yet. Sounds like next week's project for me!

Thanks again!

Ben