Poor idle after rapid deceleration

[SuspiciousPizza]

'89 GL SPFI 5DRMT. 

When I have to rapidly decelerate, for instance when a light turns yellow or I get cut off, when I get to a crawling pace or a complete stop my idle is very poor. It'll dip down to 250 RPM and my vacuum will flutter wildly within a 5-8 inHg range. If I give it a little throttle and hold it slightly above idle RPM for a few seconds it quickly subsides.

I have no issue with throttle response when I'm having these idle issues. The engine responds to very slight throttle input. It starts cold just fine unless the car was sitting outside overnight in subzero temps, then getting it started can take a few minutes and words of affirmation.

Any ideas? I'm thinking it's a failing fuel pump or maybe an injector in need of a cleaning. My fuel pump is quite noisy. I have replaced my fuel filter and the fuel that came out of the old filter wasn't dirty.

I just haven't gotten the motivation to diagnose anything in this winter weather. Ideally if it is a fuel pump, I'd like to replace it when I do my post-winter rust inspections and general maintenance in the spring.

Thanks :]

Edited yesterday at 05:26 PM by SuspiciousPizza

[bushytails]

Dirty idle air control valve (long thing with a big hose coming out the top of it, bolted on next to the throttle body), or leaky brake booster, would be my guesses.

[SuspiciousPizza]

IAC is "new" a tested good unit that was cleaned and has a new gasket. 

I initially thought brake booster too. I could always swap in a booster vacuum hose to see if that works. 

However sometimes my vacuum will flutter with the blinking of my turn signals. My fuel pump will whine at a different tone with the turn signals and my voltage gauge will dip with the turn signals as well. This makes me believe it's a failing fuel pump. In addition to the pump being fairly noisy in general. 

I do have a spare brake booster if it ever comes to that. Probably wouldn't hurt to swap that in within the coming warmer months. 

Vacuum doesn't seem to change when I'm on the brakes at a red light. I'd think if it was a brake booster, I'd be seeing vacuum fluttering anytime I apply the brakes and that's not happening. Only on rapid deceleration. 

I've also noticed the idle issue is more common on downhill slopes (when the front of the car is lower than the rear). Maybe this has to do with the fuel in the tank sloshing forward and it's causing some issue with the fuel pump. 

[el_freddo]

Sounds like your alternator isn’t doing its job like it used to. 

Try coasting down a long incline off throttle, then when back on the juice pedal watch out the rear view mirror for any blue smoke. If you run synthetic oil this trick won’t show up anything. 

If you have blue smoke it could be a number of things, mine turned out to be a broken ring land on two cylinders from memory (was 20 years ago now). Only showed a plume of blue smoke when off throttle at speed then back on it again until the oil was burnt off. The longer spend off throttle the more blue smoke produced. 

PCV system faults could be an issue too. 

The sticky IACV suggestion from bushytails is a good one to start with, as it may be slow to adjust quickly in those situations. 

If the IACV is dirty/sticky, the question then is why - failing component? Lack of appropriate voltage? PCV allowing oil to be sucked through the IACV at idle, coating it in oil, leading to gumming up over time?

[SuspiciousPizza]

I'd sure hope it's not the alternator. I swapped in a new one maybe a year and a half ago. The car has always had a voltage flutter and differing fuel pump whine with the turn signals.

After I did some electrical work it has gotten better and now drops approximately 1 volt with the turn signals whereas before it was maybe 3 volts or so. 

I can always check the alternator terminal connections and belt tension. It doesn't sound like the belts are slipping (I'm running both belts in the same pattern since I deleted my AC). 

PCV valve is a new OEM part I added when doing a full refurb of the engine. I tightened it with a flange wrench too and while I didn't torque it, it definitely felt tight. Sealing the threads would be an easy thing to do.

The engine I have in the car now was sitting for a few decades so I do suspect one or more of the valve springs are weak from the valves sitting open for so long. I didn't replace the springs because I couldn't find a full set of inner and outer springs. I'm still in need of outers, I have a full set of inners. 

Perhaps the springs just don't respond well to rapid changes in RPM? Valves get a little float? My vacuum on cold start is typically 15inHg then as the engine idles for a minute or two it'll creep up to 20inHg while staying at the same RPM (set to 1500 when cold). If the engine is REALLY cold my cold start vacuum is typically in the 12-15inHg range and the creep to 20inHg may take a little longer.

[bushytails]

Voltage fluctuating with turn signals is 100% normal.  Also, no alternator problem, except for a bearing locked solid, will significantly affect idle speed.

Valve issues are not caused by sudden deceleration.  Any issues with valves closing slowly would show up at high rpm, when they have less time to close, not at idle.

At cold start, the computer has the idle air control valve wide open and the mixture rich, like the choke and fast idle cam of a carb.  As it warms up, it needs less air to maintain idle speed.

To test the booster, stomp on the pedal as hard and long as you would for an emergency stop, while you're already stopped.  If it does the same thing, you've found your problem.  If it doesn't do it, it's probably not the problem.

My money is still on the idle air control valve.

 

[SuspiciousPizza]

I'll check the IAC tonight. It's right out front and easy to access. If it's not carboned up I'll see if there's a resistance test or something I can do in the FSM. I have a few spares kicking around I could try if it comes to that. 

Also I did do a factory PCV reroute. I know the PCV was definitely sucking oil before I did this. I lost about 1/3 quart of oil over about 1300-1500 miles. I've checked my oil level recently (about 1500 miles since the reroute) and I haven't noticed any obvious loss of oil after doing the reroute.

Then again my oil pump leaks so I'm unsure how much of the aforementioned oil loss was due to the oil pump leaks vs the PCV sucking oil. 

And my vacuum does begin to ever so slightly flutter above 3.5k-ish RPM. Less than 1inHg fluctuation but if you're looking for it, it is noticable.

Edited 2 hours ago by SuspiciousPizza