Stumbling after warm up on the road

Been a long time since I have been here. I think I forgot most of what I learned!

My 90 now has an issue of missing. It started a couple of days ago when we went out for a 14mile drive on the freeway. It was running fine and since the cap and rotor was replaced, getting great milage. Wires are high quality and good. 215K miles on the OD with the original TPS.

All of a sudden the engine started missing a couple of times and then started getting really bad. Full throttle would not really work and made it worse. Light throttle was a slight improvement. No indicator lights came on, oil was normal, temp did not change. Drove it home over 6 miles this way.

After turning it off, tried to start it. Failed. Went in the house to rest. Ambient temperatures around here have been in the lower 90's.

Went back out to start it and vroom. No problem. Took it to the Metro the next day, 2 miles, and home again without a problem. Then we decided to go out to a location 14 miles away and it came back. Turned around and made it home. It did not get worse just constant. Started and ran great this morning. At a stop light It would run fine at 1500 rpm no load.

Outside of a potential plugged cat, which sensors are likely to have gone bad that may be heat related? TPS?, CPS?, O2? ATS?

Thanks in advance.

map sensor or fuel pressure are the first 2 things that come to me.

Are all the tune-up parts fresh ? Have you tested the fuel pressure ? Have you tested any of the sensors ?

All the classic symptoms of a failing fuel pump.

A fuel pressure test is in order.

Thanks all. That was my inclination too. I happen to have an extra somewhere deep in the storage unit! The original one was replaced. The replacement can be heard when the ignition key is turned to the run position until the pressure is reached. I usually do this before turning the key to the start position. I still hear the pump but it may sound a bit different.

If I make it home, I will test the pressure.

MAP I have one of those MAP adjustment modifications from several years ago. This is the only way I can pass emissions with the Borla header and exhaust system and 50 replacement injectors. Probably have 70K+ miles on that set up. Part of the installation was a switch on the console that allows me to go back to the OEM circuit. I did that during an "episode" without an impact. Maybe I should check the wires to see if they are loose. What could go wrong with the MAP?

As stated the only fresh "tune-up" parts are the rotor and cap. Spark Plugs were checked at the same time and re-gaped. When I have the time, which is being forced on me now, I will test all the sensors per the book.

I just wanted to get your opinions one where to look first. Based on the input, I will test fuel pressure first. This is likely the culprit as there is not residual issues like smoke out the pipe.

The EGR was replaced maybe 20K ago, but will check all the vacuum lines too. Doubt that is the issue but I have be simply driving this thing for the last several years rather than maintaining it as I should.

I have another issue with the suspension of poppiing noises when accelerating and stopping that can't be reproduced parked. I suspect bushings in the control arms and springs as the noise happens both in the front and the back. Track and control arms were replaced less than 20K ago so I don't think it has anything to do with those parts. U joints feel normal too. With 215K miles it should start complaining in new places!

OK I don't drive this much anymore. The problem still exists. Fuel pump was swapped, pressure is to spec. Spark plugs look good as does the distributor parts.

I will start up and run fine then start missing.

Is there a possibility that the O2 is failing intermittently?

What about a Cat going bad?

I am going to start testing all the sensors as best I can with the DMM.

What about plug wires?

Every 30,000 miles, a 4.0 should be given new copper plugs, Belden wires, cap and rotor from Napa.

30K is a bit, uhm, proactive for wires, cap & rotor.

Uh, no it isn't.

I was Service Manager at a dealership from 1980 through 1992.
Cap and rotor were factory recommended at 30,000 along with plugs. We started to do wires at that interval because we experienced failures in them before 60,000 interval.

Do the basics, which are also cheap and easy. Make all the tune-parts top quality and new. Test the sensors, including the MAP and the O2's.

The quality of the cap/rotor/wires will dictate when they are loosing efficiency and need replacement. Plugs may work sort of beyond 30-40,000, miles but again are simple and cheap.

Cat converters have a service life of about 125-175,000 miles.

I pulled into the garage to start diagnosing the sensors. Before I started, I decided to use some seafoam using one of the small vacuum lines from the air cleaner. It stumbled as expected. The rest was poured into the gas tank.

Oh BTW. I poured some injector cleaner in about two weeks ago after a fill up.

Its been running great all day. Let it run for a couple of hours, took it out several miles without a hitch. I am thinking bad gas or slightly clogged injectors.

Here is a summary:
Fairly new Silicone plug wires (<5k miles)
Brass cap and rotor
Old but still good Splitfires - needed them to pass emissions. Stock wouldn't work
Motorsports 50 injectors (Bosch green colored)
Adjustable MAP (yes I checked that it was set for adjustable side and not original 5v - I have a switch on the console)
Borla exhaust from head to tail
New fuel filter
Bosh fuel pump (NAPA)
215K miles

Problem still exists. It use to show up a couple of miles down the road but now it it much sooner.

Could this be an O2 sensor issue?

Could be. Ive seen them do weird stuff if they get stuck. If you never have replaced it it wouldn't hurt to put on.

sjx40250 said:

I pulled into the garage to start diagnosing the sensors. Before I started, I decided to use some seafoam using one of the small vacuum lines from the air cleaner. It stumbled as expected. The rest was poured into the gas tank.

Oh BTW. I poured some injector cleaner in about two weeks ago after a fill up.

Its been running great all day. Let it run for a couple of hours, took it out several miles without a hitch. I am thinking bad gas or slightly clogged injectors.

Here is a summary:
Fairly new Silicone plug wires (<5k miles)
Brass cap and rotor
Old but still good Splitfires - needed them to pass emissions. Stock wouldn't work
Motorsports 50 injectors (Bosch green colored)
Adjustable MAP (yes I checked that it was set for adjustable side and not original 5v - I have a switch on the console)
Borla exhaust from head to tail
New fuel filter
Bosh fuel pump (NAPA)
215K miles

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Ditch the Splitfires in favor of NGK.

Has the distributer module been changed? That module can become flakey at certain temperatures and cause grief.

gdavis said:

Has the distributer module been changed? That module can become flakey at certain temperatures and cause grief.

Click to expand...

Not on a Renix, which his is. You can unplug the sync generator in the dizzy and you couldn't tell the difference.,

cruiser54 said:

Ditch the Splitfires in favor of NGK.

Click to expand...

This. Or Champion. Splitfires are a gimmick.

Try cleaning the IAT sensor.

sjx40250 said:

Problem still exists. It use to show up a couple of miles down the road but now it it much sooner.

Could this be an O2 sensor issue?

Click to expand...

Yes. But not necessarily the way you think. It is probably not liking closed loop operation (thus related to the O2 sensor) but that does not mean the O2 sensor is bad. Open loop is more forgiving.

I would consider the following due to the randomness and while hot problems:

ICM Module and coil.

Wiring, connections, to the these parts and grounds.

CPS getting too hot, cracked exhaust.

TRansmission Auto or manual? Dust from the clutch has been known to screw with the CPS signal

O2 sensor wiring, or O2 sensor

Broken Cat converter honeycomb,

Bad wiring/connector to injector(s)

But sounds like a problem in closed loop operation, or parts like the ICM overheating.