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Sudden huge problems with 96 2.2 L4 engine

I have a reliable (usually) car that started to idle very badly, stumble
and shake. I also began smelling gas in the exhaust.

-No codes shown or in CEL memory
-Checked plugs and all 4 black sooted
-Oil smells of gas
-Borrowed a scanner and STFTRM was -34
-Fuel pressure in spec when checked
-Strong spark from ignition system
-O2 upstream sensor in spec when checked
-O2 downstream sensor out of spec when checked

It was then I checked the vacuum and it was very low, (8-10 in.) and then I
heard the telltale sound of the timing chain hitting the cover. My first thought
was the timing chain or tensioner was gone, which was sapping the vacuum and
that’s why the car was running rich.

Removed timing cover and tensioner was broken. Replaced timing gears, chain
and tensioner with OEM parts. Engine is transverse mounted so it was difficult to
eyeball whether the dots and tang on the tensioner lined up, but I was pretty
sure it was lined up.

Upon start up, idle improved and mostly stable but was still off. During the
first test drive, the car stumbled, and had no power at all and got worse when
it warmed up. Pulled back into garage and no codes when scanned.

-Vacuum 15 inches to 16 inches cold
-Timing wildly changing from -2 to -26 degrees
-At load timing a solid -15 degrees
-STFTRM still at -34
-Gas smell still coming from exhaust

At this point I didn’t know what to diagnose and fix first. I figured I would
try the fuel system. Mostly because I had already started diagnosing it.

-Replaced fuel pressure regulator
-Replaced Injectors and Seals
-Checked all sensors for in spec numbers, all were good
-Checked ignition system, all parts in spec
-Completely cleaned throttle body (no maf on this engine)

Dead end. STFTRM did not change, and still running rich. So, I decided to investigate
the vacuum system next, as it was probable at this point it wasnt a fuel system fault,
and maybe it was vacuum/timing related. At this point I kinda went nuculear. I checked
or bypassed any part even remotely connected to the vacuum system I could. (sans timing)

-New OEM manifold and intake gaskets
-Removed and plugged up EVAP system
-Removed electric EGR valve and added a bypass plate
-Smoke checked engine, no apparant leaks
-Checked brake booster for leaks, none found

No change to vacuum, so decided to diagnose at load with test drives. Attached vacuum
guage to manifold inlet and ran long hose so vacuum guage dial could be taped to the
windshield, and seen while driving. Engine left overnight to cool.

-Vacuum at idle: 15 inches
-Under load (1500 rpm/10mph) but cold vacuum: 15 inches dropped to 5 inches steady
-Warm engine at idle vacuum (160 degrees): 13 to 14 inches
-Warm engine tripped SES cel P1406 (egr code because of bypass plate)
-No power under load and at 5 inches vacuum (expected)

That’s all the data I have room to give, but its the most pertinent. I want to do an
open header road test next to check for exhaust restriction but I doubt that would help much.
Any input appreciated at this point.


You gave all that information, but left out the most important information

What the hell kind of vehicle are you talking about?



Oh sorry!

Well if it helps it’s a 96 Oldsmobile Ciera. The engine is what usually confuses people, (GM used a Suzuki intake etc.) so I guess I made sure to list it mainly out of habit.

Replacing parts is a very expensive diagnostic method although I have gone to too many “mechanics” I formerly used that relied on it. ie " well , I replaced everything else, it must be the XXXX. The XXXX these days is usually the computer. In the old days it was either the carb or the distributor.

While I tend to agree with your statement, I’m not sure what relevance it has to do with this thread.

I don’t know if this helps. 06 Mustang, similar symptoms, no codes. Computer was dying.

Well it helps more than people posting anecdotes :wink: Sadly my PCM has been verified and updated earlier this year. So, it’s good to go.

Bone Daddy - Because you mentioned replacing fuel pressure regulator and fuel injectors and seals as the first step in your diagnosis.

Not at all. I think you missed quite a few paragraphs above that. All the information and readings listed didn’t just poof into my head, you know. The car has 150+K miles and the injectors were 65 dollars for the set. Cheaper and easier than having them cleaned. The regulator was looking pretty worn out when I checked it, so why not replace it with the new injectors. It’s job is to keep them equalized.

I bought 3 things that were either bad or heading that way, in a virtual sea of diagnosis steps. And even if it didn’t fix my problem, then I just consider that maintenance or future proofing. Hardly throwing parts at the car.

Perhaps I missed it in that wall of text, but after the engine started running badly again and the timing started changing wildly, I didn’t see where you opened up the timing cover again and checked to make sure it’s still in good order…

I’ll also point out that you need to be a little more scientific in your diagnostic process. I’ll give you an example:

-Removed electric EGR valve and added a bypass plate

OK, and that didn’t work, so did you remove the plate and install the EGR valve again? Because if you didn’t, you might fix the original problem later on without knowing it, but now the work you did elsewhere is causing a similar problem and so you sit there spinning your wheels.

@oldtimer_11 was a little more subtle in his comments, but what he’s getting at is that it’s great that you ran all those diagnostics at first, but when diagnostics aren’t telling you what’s wrong, randomly throwing parts at it isn’t going to help without a generous heaping of luck. Diagnostics not telling you what’s wrong simply means you haven’t run the right diagnostic yet. That’s the angle you need to be approaching this from.

Yes I have since replaced the EGR valve. I did laugh at the irony of making a wall of text comment then saying I need to be more scientific, haha.

As a non mechanic, it would seem to me that Captain Obvious would say it is running way too rich. What would cause it to run too rich? Any of the sensors that are out of whack like the MAF, coolant sensor, etc. So from a scan tool, what is the fuel trim? What is the temp sensor reading and is it accurate? Is it commanding a leaner mixture and just not able to provide it, or are the sensors telling the computer the system is too lean?

What’s the long term fuel trim? If it is also way negative like that, -20% or worse, that means the engine computer is having to cut back on the amount fuel it injects to satisfy the O2 sensor. The computer first calculates for a given driving situation the number of pounds/hour of fuel to inject based on the MAP and throttle position sensors (mostly). But to meet emissions requirements it also has to satisfy the pre-cat O2 sensor. If the fuel supply is correct after taking into account just the MAP and TPS, there will be very little O2 in the exhaust, but still a little. The more gas injected, the less O2 in the exhaust, as the gas burn up the O2; ;likewise the less gas injected, the more O2 in the exhaust. In your case the computer is seeing less O2 in the exhaust than it expects based on the MAP and TPS. It needs to do something to see more exhaust O2. So it cuts the fuel back by 34%, then it sees the correct amount of exhaust O2.

Problems that could cause this

  • Faulty pre-cat O2 sensor
  • Faulty TPS or MAP
  • Fuel pressure is too high
  • Injectors are leaking or rated incorrectly for the engine, rated too many points per hour in other words.
  • Ignition timing off
  • Valve timing off
  • Faulty engine coolant temperature reading
  • Faulty ambient air temperature reading

I’d start with measuring the fuel rail pressure.

All that info is literally in the original post amigo.

The warm idle intake manifold vacuum of 13-14 in Hg definitely seems low. If the ignition and valve timing checks out, might be time for a compression check.

LTFTRM was 1.7 or so.

Yes I’m aware it’s running rich. and the process that causes it, but you explained it well nonetheless.

-Upstream sensor good as indicated in original post
-TPS good as indicated in original post
-Car has no MAF as indicated in original post (google the L4 2.2)
-Injectors new and good as indicated in original post
-ECT sensor good as indicated in original post
-IAT and IAC good as indicated in original post
-Fuel pressure good as indicated in original post

As far as valve timing goes possibly the cause. However I doubt -34 fuel trim is a one tooth off problem, even though no power on load indicates just that.

I would guess that because everything ran well before the timing tensioner went out, that this is where the problem still lies.

If it was a car I fixed the timing on, I would have gone back and rechecked my work when I first started it up and it still ran poorly.

I didn’t read that you pulled the timing cover and checked the timing again.

Maybe I missed that part.


My first thought is it’s not lined up. you’re still off a tooth.

The long term fuel trim looks pretty good, it’s only the short term that’s off. hmmmm … that sort of makes me think this isn’t a systemic problem like a faulty sensor, too high of fuel pressure, wrong injectors, etc. The engine isn’t holding a steady air/fuel mixture seems to be more the problem. Suggest to look for ignition system and valve timing problems of an intermittent nature. Loose timing belt/chain, faulty ignition module. If you have any doubts at all about the valve timing and the way it is set up, start there. That has to be correct otherwise the engine will never run correctly. Crank and cam position sensors should be suspect too. As well , compression.

Are you sure your engine doesn’t use a MAP sensor?